Because they didn't. You seem to be very misinformed about German economics in the 1930s and '40s, but far from being "debt free" the entire thing was financed by massive amounts of debt that Germany had no way of repaying... except through conquest. The German economic "recovery" of the period was entirely based on the expectation of territorial expansion to the East, the new acquisitions from its neighbors, and the subsequent exploitation, paying for the unsustainable spending prior. More of course can be said, but this is covered extensively in our FAQ which you can find here.