They just bit off more than they could chew. At the start of the war they had plenty of manufacturing capability. They just lost a lot of it because we could bomb the crap out of them. That's why we could build rifles faster. They couldn't touch us. We could bomb the crap out of them. So air superiority could be said to be the reason we won the war. We shouldn't forget that much of what we built is at the bottom of the Atlantic because it didn't make it to Europe.
Germany's war production actually increased as the war progressed (until right at the very end). Their problem was they tried too build to many different types of weapons. When the war ended, the Allies found one plant still building an engine for a Bismark class battleship. They didn't have any ships under construction to put it in.
The war was over before the US ever landed at Normandy. Russia took 20 million dead (military and civilian), but destroyed the German army in the process.
I heard an interview of a German officer after the war describing fighting the Americans at Normandy. The officer manned an anti-tank gun in the hedgerows just off the beach. When asked why the Germans lost the war he said that every time an American tank entered the hedgerows, his crew destroyed it. Another tank would enter and they would destroy it. Unfortunately, he ran out of shells before the Americans ran out of tanks.