Part of the issue is, a lot of developers just aren’t inclined to use the extra power. It costs development resources they’re not guaranteed to earn back. And, their idea, while unique, isn’t reliant on ultra-realistic visuals. In some cases, it’d be harmed by them.
We even see the PS4 stick around as a “low cost option” for some players. It can’t run the rare PS5 exclusive, but it will still let you join for significant games like Fortnite, etc.