These days websites can be games, drawing applications, video players, etc. As a result, browsers have basically become operating systems. In addition, the browsers try to support even the most horribly written websites, but that means more bloat in the browser. Meanwhile faster computers mean that people developing websites are just doing more and more javascript, more and more animation, more and more mouse tracking, etc.

If you have an old device with an old browser, a lot of modern websites are completely unusable. I have an old iPad that's too old to update, and it's not actually possible to use browse Github anymore. It just ends up with javascript elements on the page that never finish loading. And Github isn't some site thrown together by someone vibe-coding their first website or something.