It was a combination of factors. One of the main ones was that beach bunkers could withstand significantly more damage than anticipated. Even a direct hit from the gun of a destroyer offshore did little more than rattle the defenders. If you go to Juno Beach, you can actually see a bunker that was hit with a shell; there’s a good foot deep crater in the reinforced concrete, and otherwise it’s more or less fine.

Another problem was the coordination. There was no instant satellite chatter. Radios were extremely cumbersome and heavy, and their operators were priority targets for snipers. The operation required coordination between naval boats, landing craft, the airforce, wave commanders, and SHAEF (and keep in mind that even though Omaha Beach was an American effort, and certainly the most difficult beach, Canada and Britain each had significant invasion forces in play, meaning that communications had to be disseminated to the appropriate chain of command of multiple countries with different army structures).

The weather also wasn’t good. Ideally, landing craft would have been on the beach immediately after the bombardment; initial plans called for men to have disembarked almost instantly. However, the seas were rough and not at ideal tides. Those had been two days earlier, but poor weather prevented the landings, and D-Day itself was only given the go ahead mere hours beforehand and only because the alternative was a three week wait. Because the weather was not ideal, many of the landing craft got lost, were delayed fighting the high waves, got caught on mines or anti-landing obstacles (such as Czech Hedgehogs), or were outright swamped. This is particularly true of many of the amphibious tanks which were to provide heavy fire support on the beach. Many of them sank however, significantly reducing the ability of Allied forces to continue pressuring pillboxes and bunkers.

Essentially, plans called for the bombardment to continue until immediately before men disembarked on the beach, and also called for tanks and artillery pieces to be on the beach supporting troops. However, real world factors including poor communications, unexpectedly strong German positions, and unfavourable weather significantly complicated the situation and prevented the effective continual bombardment of the beach.

In addition to the good response by u/canadianstuck describing what actually occurred during the D-Day landings, there have been significant doubts raised about the veracity of both volumes of D-Day Through German Eyes, as well as other books by Sprech Media like The Last Panther, and even the "Sprech Media" company itself, despite the books' popularity (see the Amazon listings for both books), by both casual readers, historians, as well as r/AskHistorians users such as u/commiespaceinvader. I will direct you to their earlier answers which point out these doubts:

• What is the opinion on "The Last Panther" and "D-Day Through German Eyes" by Sprech Media? Fact or Fiction?

• "D-Day Through German Eyes" Book Two, by Holger Ekhertz final chapter contains a reference to a thermobaric weapons code named "Taifun B". This is the only reference to any such weapons I can find. Was it real?

Over confidence?

Eh, rigid overplanning is a better word, since Operation Neptune - the naval side of D-Day which included the bombardment schedules - ended up congealing into this monstrous 3 inch thick ops plan that didn't account for any possible deviation from it, with near disastrous results until some largely forgotten extraordinary heroism saved the day.

As much as it's breathtaking to walk around Ponte du Hoc nowadays and see the craters from the 14" shells of the battleships, as /u/canadianstuck points out, the shelling itself was largely ineffective in its actual objective. This isn't terribly surprising; as the Marine Corps found out the hard way, 'beachheading the Japanese for years' with even more potent bombardment generally didn't succeed either. That worked only as long as the latter hadn't yet figured out that opposing initial landings on the beach wasn't the best use of their forces. Once they did so, they simply crawled deeper into their caves, came out to fight after the shelling stopped, and created legendary bloodbaths. Even the contemporary stuff published immediately like after the war like Sledge noted this.

Presuming the source material is valid here - and as /u/the_howling_cow and /u/commiespace note, it probably isn't - it's worth noting that 4 years of experience with history's most powerful artillery barrages terrifying troops proved largely useless when they were properly fortified (versus creeping barrages during advances, which did work to a degree). This is one of several reasons why the ops plan here was somewhat flawed to begin with. Besides things like screwing up the amphibious tanks, most of which promptly drowned their crews at the bottom of the channel given terrible preparation, it assumed the initial shelling would work despite plenty of evidence it wouldn't, and had no backup plan if that was the case.

What won the day? At about 830 on D-Day, recognizing the peril on Omaha that had gotten to the point where Bradley strongly thought about withdrawal, about a dozen destroyer commanders took it on their own initiative (albeit with their division commander rooting them on once he figured out what they were doing) to get within 1000 yards of the beach, close enough to visually target German positions on Omaha and Pointe du Hoc.

For the next 90 minutes, their actions probably saved the landing. The best accounting comes from Craig Symonds' chapter on it in Neptune:

"The destroyer captains responded with enthusiasm—indeed, almost too much enthusiasm. Most of the ships were American Gleaves-class destroyers that drew more than thirteen feet of water, and the gradual slope of Omaha Beach made close-in fire support extremely hazardous. [Depth was estimated at 12-18 feet.] It was self-evident that if a destroyer grounded in the shallows, the German gunners could blast it to pieces at their leisure. Nevertheless, they now came speeding shoreward at twenty knots or more into water that was both shallow and outside the swept channels. One sailor on an LCT approaching the beach was shocked to see “a destroyer ahead of us with heavy smoke pouring from its stack.” To him, the ship appeared to be out of control and headed directly for the beach. My God, he thought, they’re going to run aground and be disabled right in front of the German artillery. At the last minute, the destroyer made a sudden hard left, turned its starboard side parallel to the beach, and began “blazing away with every gun it had, point blank at the defensive positions.” The sailor was thrilled to see “puffs of smoke and mounds of dirt” flying “everywhere on the hillside as the destroyer passed swiftly by.”"

Where there was coordination with the ground like at Ponte du Hoc, the destroyer fire was even more devastating despite being indirect and relying on spotters:

"Hard pressed by German light artillery, the Rangers used a blinker light to send target coordinates to the destroyers offshore. Satterlee responded first, joined by Thompson at 8:30 and by Ellyson (DD-454) an hour later. Unable to see the target, the destroyers had to rely on indirect fire, stopping after each salvo to receive a report from the Rangers about the fall of shot. At 9:52 the Rangers called for the destroyers to cease fire, and at 11:30 lookouts in the destroyers saw an American flag flying over the position."

But even on Omaha with communication in chaos and and effective camouflage for the Germans, it still worked as suppressing fire even early on:

"The other destroyers of Sanders’s squadron, operating off Omaha Beach itself, had great initial difficulty identifying appropriate targets. The poor visibility along the beachfront and the excellent German camouflage made it all but impossible to figure out where the German guns were. Some of those guns retracted into underground bombproofs; others were so well hidden that, as one sailor put it, “you couldn’t see it if you was ten feet from them.” In theory, the ships were supposed to coordinate with shore-based fire control parties that would identify the targets and report the fall of the shot, as the Rangers did on Pointe du Hoc. But there was such chaos on the beach that morning, and such a dearth of working radios, that not until the afternoon did the destroyers establish regular radio communication with spotters ashore. In the meantime, the destroyer skippers were compelled to seek “targets of opportunity.” There was one immediate benefit of their arrival, however: many of the German gunners, fearful of disclosing their location, briefly held their fire, while others shifted their fire to target the destroyers. In both cases that created a blessed, if temporary, respite for the soldiers on the beach."

And then turned into far more:

Compared to the aerial bombs dropped from high altitude and the big shells fired from ten or fifteen miles offshore, the smaller-caliber destroyer fire was more accurate and therefore far more effective. The bigger 8-, 12-, and 14-inch shells from the cruisers and battleships had made the ground shake, but they had left the German gun positions largely intact. Now those positions were pounded with hundreds of 5-inch shells and knocked out one by one. One witness recalled seeing three 5-inch shells hit within twenty inches of a narrow gun slit on a pillbox, and in at least one case a German artillery piece was hit directly on the muzzle and split wide open. A beach master on Omaha watching the tin cans fire into the cliff later claimed, “You could see the trenches, guns, and men blowing up where they were hit.” There was no doubt in his mind that “the few Navy destroyers that we had there probably saved the invasion.” [something which Bradley also acknowledged after the war, where he stated “the Navy saved our hides (at Omaha).”]

None of this was remotely anticipated in the op plan. Even during the successful destroyer attacks, it still got in the way: at one point, destroyers were reminded they were supposed to stop at 50 percent of ammunition as per Operation Neptune and turn back to England to rearm. They ignored the 'suggestion', and in one of the greatest contributions of tin cans during the entire war, probably saved the landing even if they've never really gotten credit for it by most authors or in popular culture.

That's the long version of why shelling with the big guns for a few hours more wouldn't have mattered an iota: it didn't work and wouldn't have if it had continued, but improvisation and bravery and smaller caliber weapons did.

I'm not saying that the American should keep shelling the German as they were inserting their troops in. No. That's stupid. What i'm saying is, or rather asking... what's the hurry? Why not an hour of shelling before insertion, so that the beach is clear, both of the Germans and obstacles? That way you'll get a easy foothold. And It's not like it was your first rodeo? You've beachheaded the Japanese for years using this tactics. Why the sudden change in M. O? Of course, the German would reinforce some distance behind the beach and would be ready to drive the Americans back. But hey, they would still do it regardless.

I also want to touch on this real quick.

The USN was in a far better position to stand off an island in the Pacific without serious fear of counterattack. Japanese naval strength had been pulled back by the summer of 1943, and no surprises were going to sneak up. meaning picket forces could extend day and night, and while the fleet was far from safe, it at least knew the type fo threat it faced, usually night strikes by bombers and submarines. The cramped, rough waters and narrow weather window of the English Channel provided no such niceties. They had to go when they could. And enemy counter thrusts would provide much shorter warning time than clockwork nighttime air strikes or a long massed sortie by Japanese naval forces.

And of course there is the matter that even extended bombardment was not a sure fire answer. Nor were all the landings preluded by extended bombardments. And the larger the island/landing space often the harder it is a job, as additional strong points, areas of concentration, and infrastructure call for attention. Arguably the Allied air and naval gunfire support on the actual morning of June 6th was secondary and less important than the total working over Northern France received in the weeks before and after, exacting a tool on any German forces attempting to move.

While in the Pacific, landings which predated NEPTUNE were not always drawn out. One fot he first of the new Fast Carrier Force and Amphibious Train came in November 1943 in the Gilbert and Marshall islands and Tarawa in particular featured relatively short 1-2 hour bombardments. While others like Kwajalein in January 1944 had evolved some, including closer ranges, mixed AP and HE shelling, and even trying to seize secondary islands to use field artillery! But was still just a short furious morning before the troops hit the beach.

And the same week that NEPTUNE kicked off, US forces landed on Saipan in the Pacific. It did feature a 2 day series of naval bombardment on the larger island by a tag team of newer fast battleships, and older ones in turn. But even that wasnt enough to simply secure the beach in the face of careful Japanese preparations. And just as ammunition was a critical concern in Europe, it was doubly so in the Pacific with not just shells, but other consumables like fuel and food, limiting how long any bombardment could last. It would not be until 1945 and the final maturation and development of the fleet train and a series of major forward bases that the fleet could truly stand off an island, and even Japan itself and commit to fire missions without worry about resupply.