How do you explain PS4 games looking better as the generation went on? You honesty think they knew about to eek out the performance to make GoW or Red Dead or TLoU2 look as good they do from the start? Even though it is a x86 platform it takes time to figure out all the tips and tricks to mitigate the consoles shortcomings. Though i will concede that they probably got out the blocks faster on the PS5 as they don't need to learn a whole new API's and such
There wasn't that much of a gap, Infamous SS, for example, looked very impressive at launch and it was open world. Both it and Shadowfall I've recently tried on PS5 and both hold up damn well with Infamous being particularly impressive with effects - a PS4 launch title, mind.
There are just games that look better than the rest because they're super polished and as you've probably noticed, they aren't exactly beaten by every new release so the devs aren't consistently tappng into anything. It's just a case of a few special exceptions and talented devs. In fact, it's just the opposite, most newer games have increasingly more performance problems because of various reasons.
Just look at PS5 launch titles, Demon's Souls is still one of the best looking games on the platform and it's been three years.
Similarly, find me a game that looks significantly better than RDR2, and that's a 2018 PS4 title.
It's all a matter of a good engine and there's just not that much to tap into or figure out about the PS5 at this point. We might get better stuff but it won't be anything groundbreaking.
Also, we're not at the start but three years into its lifespan, with rumours of a Pro despite all the forum experts here saying it'll never come, and the number of games that can't hold 60fps with upscaled res or just default to 30fps is increasing and not decreasing, which should tell you something.
SMALL EDIT: It might sound like I'm really negative but I'm very happy with how the games look, I just wish devs focused on performance and didn't try to go overboard on hardware that just doesn't cut it for what they're trying to do, like RT etc.