Where have all the STM32H750VBT6s gone?

Someone ahead of you in the store bought all available stock. No more doughnuts today..

They may have back-orders against stuff for delivery in March, and August..

People on the forum have been complaining for the best part of two months, coming to the realization of what a chip shortage looks like. Try a Google of "global semiconductor shortage". And then what a 26-40 week lead-time means in terms of planning early, and getting an order on the books as delivery pushes out further.

https://www.gizmochina.com/2021/02/14/asml-vp-says-global-car-chip-shortage-an-indication-wider-demand-issue/

And as stock runs low, many people do panick buys, so stock runs out even faster!