Randomly rewarding the latest 'feature' so you get a load of shorts of videos you've already seen, random votes for things, etc.
Zooming out there's typically an ecosystem benefit, because otherwise why would they do it. That ecosystem benefit isn't always about money, if no one watched videos there would be no business. In fact a business that is often much better than a business with no users that isn't, so user happiness and experience tends to be prioritised highly in products/companies like this.
A video with 1000 upvotes and 2000 downvotes tells me something.
A video with 1000 upvotes doesn't tell me anything.
I can still downvote, I just don't get told the downvote total.
If they decided the system was broken and decided to replace it, I could understand. But they haven't changed the system, they've just removed the utility it did have.
You think it's "pretty good" when a video is interrupted at some random point in time (usually while someone is mid-word) to play some annoying video at a much, much louder volume than the video? That is not what I consider a "good user experience" at all.
Other than ads, which is the business model and keeps the content flowing... does YouTube not care about UX? I generally find the UX pretty good. I can always watch a video near-instantly, on nearly every device I own, search is astonishingly good even when my Apple TV mis-hears my searches, and it's one of the biggest video streaming sites in the world.
As a user I think the UX overall is pretty great. Sure there are things I'd tweak, but I can see reasons for them.