Because the area that was covered by the Atlantic Wall was massive, and two other fronts were simultaneously calling more immediately for resources.

The coastline that Germany theoretically had to defend from the Spanish Border to Norway was about 3,000 miles, while the majority of German production and fighting forces were engaged in the deadly struggle in the East, along with the sideshow in Italy. Even the moderate rebalancing that did occur could never have allowed Germany to field defenses in strength everywhere. And while Normandy was a higher priority than say, the Bay of Biscay or northern Norway, it still was not as high a priority as other sectors like Calais.

Now some searching does not lead a hard number on artillery tubes churned out by Germany through the war but the general consensus seems to be around 130-160k all told. But not all of that would have been in operation in the first half of 1944, or serviceable, or useful in defending from a landing. But for a nation fighting in as many places as Germany at once that leaves something to be desired, Britain produced only slightly smaller numbers for instance, and was able to supplement it with Lend-Lease aid from the US.

Seems to me like it would have been a really good idea for the Wermacht to have the beaches of Normandy zeroed with insane amounts of artillery.

First of all your gravely underestimate the resources used in Verdun shelling. But this is understandable. Some numbers for you.

On the first day of Verdun the German Army fired about 1.000.000 shells on a 30km ( 14 miles ?) strip.

Let us compare that to World War II. In the entire year of 1944 all fronts the Wehrmacht fired about 45.000.000 field and heavy gun shells. The entire year all fronts. Most of it against the Red Army. You might notice, the amount of shells used for Verdun was ridiculous. We see how important the shell production was. Many historians often list numbers of guns for strength comparisons but the important part is the shell consumption. Most German units, in Normandy for example, were rarely able to fire at full capacity even during the heights of battle. There are many testimonies of soldiers remarking on the fact that the Allies were firing massive salvos even outside of combat. German artillery units were saving the ammunition for high priority targets or during direct combat.

Taking your proposal we assume the Normandy area, the Calais area, Belgium and some other coastal strips get supplied with a "full Verdun" allocation of shells. This is up to 500 Kilometers of coastline. We not even include the Bretagne. They would have to stockpile 15.000.000 shells ( very rough estimate ) just to sustain action from the landing until afternoon. That's a third of the field gun consumption of the entire Wehrmacht laying around. If you consider reinforcing the Netherlands or Northern Germany it gets even more unfeasible.

To my knowledge, the German Army fired more than 20.000.000 artillery rounds during Verdun. A mind-boggling number indeed.

Some interesting side info, during the last year of the war the US artillery consumption in Europe was comparable with the entire Wehrmacht while fighting only a fraction of the Wehrmacht forces. That is how you win wars.

Germany in contrast to the Western Allies had an arms industry which operated under a good amount of shortages. The defense you have seen in Normandy is a direct result of that. Concrete and sadly workers ( Zwangsarbeiter ) were kinda expendable. Steel was not. Steel was one of the most valuable resources in WW2 and Germany's production was limited. Germany allocated those resources quite wisely, in the grand scheme of things and focused a good amount of steel on one of the most important weapons: Artillery more precisely artillery shells. One of the major reason for the casualty disparity on the Eastern Front. The correct allocation of steel was therefore of utmost importance.Everything produced was more or less immediately allocated to the front or for new units. Your proposal would require production increase which was not possible and if it would have been possible those shells would have gone to the Eastern front.

Other points:

The coast was defended by a good chunk of artillery anyways but most coastal strips were rather short and the main defensive focus directly on the beach was on mortars and machine gun nests. A successful attack would see the opposing forces only spending a short amount of time directly on the shores. Ohama beach is more and exception than the norm. The first defense lines were at some places compromised within the first hours. The attacker chooses the when and where and therefore will pick scenarios he considers favorable.

Existing artillery batteries were scouted and bombed anyways to with varying success.

But while I said all of this I didn't mention the most important part, if the Wehrmacht would have created such defensive measures the Allies would have just adapted.

The German defensive strategy was to use flexible Panzer units and crush units before they could establish a functioning bridgehead. That didn't pan out either obviously but it was most certainly a cheaper and better option. The areas in Northern France and Belgium were often chosen as places for refitting units or units which got newly raised. In case of a landing those units were ready to strike within hours depending on the landing locations. Quite interestingly this concept would have worked if the German units were handled properly. For example, the Eastern part of the landing zones ( Sword, Juno) could have come under a concentrated counterattack of two Panzer divisions, one of them fully equipped and "elite", on June 6. The opportunity costs of this was a fraction of the artillery option.

Would your proposal would have worked? Pretty sure it would, landing on such heavily shelled beach would likely result in failure but the important part the Western Allies would have choosen another path to victory. In the Eastern front the Red Army would steam roll the Wehrmacht even harder because the ground forces are deprived of one of the their strongest assests their artillery arm.