Here’s a simple reason, briefly.
Germany’s all-in commitment to military power crippled its economy, so the rich reserves captured from newly seized land had to pour in constantly; otherwise, the army and arms industry would become unsustainable. Hence, Germany had to expand to survive. However, as it did, its land grew, so Germany needed even more army to keep its land. Also, the richest countries got conquered, so conquerable rich countries grew scarce. Germany would thus run out of invasion opportunities and would have to rely on its economy only, which was drained, though.
A good analogy is the Mongol Empire, which also used expansion to power its army’s growth (eg. it used siege weapons previously captured from China to seize Arab cities). Like Germany, the Mongol Empire couldn’t sustain its vast territory and army if it couldn’t constantly invade rich regions and steal their wealth. After it stopped growing, the Mongol Empire gradually dissolved, just as Germany did.
