There is a history of trying to support context switching in CPU hardware. That was a hot idea decades ago. Intel had call gates and some other stuff. Some of the RISC machines had hardware to spill all the registers to memory in background. Some early machines, from the days where CPUs were slower than memory, just had a context pointer in hardware. Change that and you're running a different thread.

None of this helped all that much. Vanilla hardware won out. This is to some extent a consequence of C winning. C likes a single flat address space.