I am assuming the OP meant the Atlantic Wall fortifications here..

The short and simple answer is that it was 1944. The attritional battles that had begun in earnest with Barbarossa had sapped both German material and manpower dry by the Overlord operation. The Wehrmacht had suffered losses that it could not replace. Both the human and material capital were in increasingly short supply and both reinforced each other. For example, the air resupply efforts of both the winter 1941/42 pockets and the Stalingrad combed out instructors and training aircraft to meet demand. This ripped the guts out of Germany's air transport arm as well as hobbled any efforts to rebuild it. Speer's so-called "production miracle," which was not all that miraculous, had stabilized the situation of resupply somewhat as did a more thorough effort to mobilize German manpower, but Germany was operating in the red by 1943. Rebuilding and reconstituting broken formations took men, material, and time. All of these factors were in short supply for Germany.

The Atlantic Wall was a microcosm of these problems. Outside of a few key areas, many of its fortifications were incomplete and understaffed. Organization Todt had managed to repurpose and ensconce a large degree of captured weaponry in its ferro-concrete bunkers, such as the battery nicknamed "Big Willie" by the Allies which used 13.4" guns salvaged from a scuttled French battleship to guard the port of Toulon. But such Beute weapons were reliant upon captured stocks of foreign ammunition. The Germans could have perhaps reopened captured production lines earlier in the war, but this was a road not taken. Much of Germany's occupation policy was predicated on serving the needs of the Reich first, which often meant seizing machinery and tools, carrying them back to Germany where they often languished gathering dust because the Germans lacked the native manpower, organization, metals, and shopspace to use them efficiently. Thus the Germans often spread out this non-German weaponry and their ammunition.

German weapons and manpower were also in short supply in 1944. Some of this was the Germans' own fault as they reinforced some areas too heavily such as the Channel Islands, which were a complete waste of resources. But for the most part, the Atlantic Wall was trying to do too much with too little. American popular history accounts of D-Day almost invariably describe the Omaha beach defenders, the 352nd Infantry Division, as "veterans" or some other qualifier. But what they leave out was that while this division was more combat-capable than others on the line in Normandy, the 352nd had been formed from divisions shattered on the Eastern front as well as recent recruits. Many of the latter were malnourished and incapable of performing an extended forced march; sitting in its bunkers was about all the 352nd could realistically do in June 1944. The only panzer division to engage the Allies on D-Day, the 21st Panzer, was likewise in rough shape. This division had been litterally destroyed in North Africa and outside of a few key personnel, had to be reconstituted from scratch starting in 1943. Like the Atlantic Wall, it too had been augmented by captured French weapons, and while the armored vehicles produced by Baukommando Becker tended to use German guns, their mechanical reliability left much to be desired and they were a logistical nightmare to maintain over the long-term. The increased German production managed to also feed into these units, but their replacement weapons had significant quality-control problems and other issues. The declining quality of German weaponry was one of the facets of Speer's miracle as German industry increasingly relied upon ersatz substitutes for strategic metals and minerals. The complete Allied air superiority added a further problem as it made resupply of units in the field even more difficult as Allied airpower destroyed the transportation grid and made a total mess of German resupply efforts by road or rail.

These problems were one thing, but the Atlantic Wall covered an area from Scandinavia to the Pyrenees. There was just not enough manpower or weapons to cover this much ground. The Germans judged Normandy as a medium-risk region. It was not very proximate to Germany or northern France and an Allied invasion there would need time to build-up to retake France. So the Germans planned their defenses accordingly and with the means available. It might look foolish in hindsight, but there really were not that many other options available to them. They could have beefed up Normandy's defenses from other garrisons, but this would have left these sectors vulnerable. The German ability to reconstitute shattered formations and resupply them was fraying by 1944. The Wehrmacht could have arrayed its defenses more sensibly in 1944, but a realistic counterfactual scenario would have likely ended the same way. The Germans were trying to do too much with too little against an opponent that did not have the same handicaps