I mean they had under their control most of Europe - one of the richest, most developed and populous regions in the world.
Several reasons
When the Nazi's invaded Czechoslovakia, they immediately assimilated their arms industries. Wise move, because its doubtful if the Germans could have staged the invasion of western Europe without all those Czech arms.
But when they got hold of western Europe, they preferred to loot and pillage the industry of the occupied countries instead of turning those industries to their advantage. For example, France was one of the largest producers of arms in the world in 1940, yet the Germans preferred to loot machine parts as they did works of art, rather than committing everything to the war effort.
By the time they clicked that they needed more stuff, it was too late. All those machine parts had spent the last couple of years rusting in a railway siding somewhere in Germany and were now useless.
What's more, instead of being nice to the people in the occupied territories, the Nazis behaved like the depraved nihilistic murderous scumbags they were, and preferred to alienate and enslave the populations of those countries than convince them to join their crusade against "Jewish Bolshevism"
Sure the SS managed to recruit the odd division from Scandinavian, but this is an order of magnitude less than those countries should have been able to field by themselves in peace time.
Europe depended on trade for its prosperity, as it still does, and so does everywhere else.
However, the Royal Navy rather neatly stopped the Nazi's from trading on the open market for whatever it needed. Need some oil? Tough titty mate, Maybe your friend Romania can sort you out for a while, but its not going to see you through a long war.
Need some tungsten for armour, APDS bolts or machine part alloys? Well you can try and get some from spain, but the US and the British and buying all the tungsten they have, driving the price up to ludicrous levels.
If you cant trade on the open market, and you have two massive economies driving up the price of any strategic material you might be able to purchase, then you are in deep trouble!
The Nazis were terrified of the memory of 1918, when economic hardship pushed the country over the brink of the revolutionary precipice. Not until it was far too late did the Nazis put their own economy on a proper war footing for fear of fomenting revolt on the home front.
Yet manpower does not simply pop out of the ground like garden gnomes; they can have all the land they want; but a combination of unwilling or incapable conquered nationalities, racial shortsightedness and a small German population meant that the average amount of men reaching military age annually would never compete with the power and co-ordination of 3 major nations such as the USSR, US and UK. The Germans foolishly fought a multiple front war and suffered as a result.
Your opening premise 'they had Europe under their control' also implies that the areas conquered were suddenly and immediately assimilated for complete exploitation. Given the fact that the Germans had to often commit entire divisions to anti-partisan and security activities, we can find a flaw in this reasoning. The Germans could not exploit the resources to such an extent so quickly and they'd without a doubt have needed a peace and some amount of years to fully incorporate their conquered territories - the ones they actually wanted to incorporate into the Reich, that is.