In a World where virtualization has allowed to run several virtual machines on one physical machine (not even high-end) for 15+ years, it looks like a strange concept to need one physical piece of hardware per VM.
I actually understand why it makes sense, simplification and best performance for GPU intensive context, yet it feels like "let's virtualize, not really".
The real reason is money, as it usually is. Nvidia and AMD GPUs can in fact split their resources per VM - if only you buy their enterprise/research versions of their cards, like Quadro and Firepro. Most people have no need of VM GPU passthrough, so they don't support it for consumer cards. Intel does because they are not really in the graphics space (not yet anyway, we'll see how Xe does).
I actually understand why it makes sense, simplification and best performance for GPU intensive context, yet it feels like "let's virtualize, not really".