The raw resources you need to build robots are constrained and have a very very long and complicated supply chain.

In the second world war, Germany had impressive science and technology, but, as Stalin said, quantity is a quality in itself, and The Third Reich found out that it was an impossible task to get the metals and fuels for their advanced jet engines, rockets and U2 boats, when between then and those resources there were masses of tens of millions of under-equipped and malnourished soviet soldiers fighting what was an existencial war for them.