Most games render frames of a 3D world in less than 17ms, but most websites take 3-7 seconds to load because of all the ads and bloat, and things shift around on you for another 20 seconds after that, so when you go to tap a link you accidentally tap an ad that finally loaded under your finger. If you optimize those websites, they run super fast though, but it's quite a pain to do with the dependency bloat in modern tech stacks...
(note: games lag as well when you drag in a million dependencies you don't need)
The thing is, most sites and web apps try to solve a user problem, and if you are the only company in town that solves that problem, then performance barely matters - what matters is solving the problem. The users will put up with some pain because the problem is even more painful.
With games, it's all about the experience of interacting with the software - so performance is (hopefully, depending on your team and budget) amazing.
That, and... performance tuning is hard work, and I think most people don't know much about it. It's a fractal rabbit hole. Cache misses, garbage collection, streaming, object pooling, dealing with stale pointers, etc. Even I have a ton to learn, and no matter how much I learn, I probably still will have a lot more to learn. It's easier for many teams to hand wave it I guess as long as they aren't losing too many customers because of it.