Earlier quoted context omitted.
They get hate because they are almost always annoying and distract from the actual content. Nobody cares how creative your web page design is if it sucks to use so badly that a plain text page would be better.
But do they really suck to use so badly? Like for real? The examples that have been listed in the comments (Apple and Claude sites) look totally fine to me. People here are talking about them like they are the worse websites ever.
https://www.apple.com/macbook-neo/
Try to skim this page to get a sense of how much information is on the page. You can't. 90% of the scroll time is stuck in useless animations.
Every time you scroll down you have to wait for the page to render. We've somehow recreated the dialup experience with single page apps.
Your brain is trained on how scrolling works on 95% of pages. Breaking that patterns causes tons of cognitive overhead. You now have to do a double-take every scroll action when you just want to absorb the contents of the page.