I'm playing the game nicely at 30FPS. With that said, part of the allure for me is the cinematic experience, and not just in cutscenes. I like those moments when I'm street-level or slinking through a high-tech office.
It's hard to say why Insomniac did what it did. There's no guarantee the ray tracing would scale well to different resolutions while hitting 60FPS (I'm not ruling it out, just noting the possibility). It might also have been a matter of simplicity: this reduces the number of variables, makes it easy to understand for non-techies and guarantees consistency. You won't watch your game drop from 1440p to 900p just because there's a lot going on.
You're likely right about optimization. That Unreal Engine 5 demo by itself made clear that there's a lot of room to breathe. This has been true of a lot of games during console launch periods, it seems — there were titles in the PS4/XB1 era that didn't feel like huge leaps over their PS3/X360 counterparts.