Have you looked at Youtube comments recently? It's a mess. There's no organization, half of the replies are censored but are still being counted, so you have plenty of "1 Reply" links but nothing shows up when you toggle. Half of the top level comments are completely useless comments that are only posted to help the creator "beat the algorithm", every successful creator now has a call to action to write a comment.
I see two options: a) Google can't do any better than that, b) Google has a reason to keep it in the current state (I'll put "Google doesn't know because nobody at Google has used Youtube in the last 5 years" and similar options under "a").
a) sounds ridiculous, b) sounds conspiratorial. What are the other options?
And again, I'm not saying they are making search worse on purpose (no "from now on our core mission is to make search suck"). I'm saying they aren't optimizing for SERP quality. They seem to care about index size (maybe it's an internal KPI? would certainly explain their aggressive guessing at additional URLs that you might have on their page but don't link to, don't add in sitemaps etc, and their stubbornness in keeping results from the index even if they've been 301ed or 410ed ages ago (they do get downranked after a while though)), but I assume that they mostly care about paid ad clicks, and if something increases ad clicks while the result quality decreases, they'll do it.