I don't know. Maybe because it's always a tremendous effort for Apple to secure support from those guys considering how few they actually sell? All I'm saying is that I think they consider it to be worth their time and effort. Mac OS and Apple Silicon will be optimized for each other on every front. I'm not putting it past them to absolutely aim for Mac Pro-level integrated GPU's.
After all, they did just put all their effort into a completely redesigned, Intel-based (and very expensive) Mac Pro that is going to be very short lived.
I think the danger will be in ignoring Apple's advice and
assuming they don't have the chops for it.