Beyond conspiracy theories it's interesting to speculate why Google is not providing native search-in-subtitles and search-in-comments. The easiest explanation is that they don't trust the quality. They probably tried it and reached the conclusion that it doesn't improve search in any meaningful direction.
I know from experience that search in user reviews is very hard. Unless you really understand the review (which was tried via sentiment analysis) you cannot rank results well. But now with the new LLM models I think it would work better.
I’m pretty sure google uses the captions in search. As long as you search from within YouTube. I regularly search for keywords and find hourlong videos where it’s mentioned somewhere in the middle and nowhere in the description.
I still have doubts. Google uses anchors heavily in search, so it's hard to prove if a result is due to matches in captions or in a website linking to the video. Would be good to find proof one way or another.
I suspect what you see is the search "learning" from the users -- someone is looking for a specific video that they've seen before but need to refine the search terms a few times before they find it. Thereafter, Google remembers the entire sequence of search terms and associates them with the video even though none of those terms might actually exist in the indexed video metadata. I've noticed that Google Search at least does this very obviously.
I mean Google search does it obviously using page rank which is that if someone links to the page with that word it uses it. Me I was searching for arcane words that I remembered hearing in the middle of a podcast. I doubt anyone actually linked or searched for the same with relevance to that podcast. Also the solution you're saying needs far more intuitive and intricate development than just indexing captions lol.
I think Google is now thoroughly infected with Big-Company-itis. A couple departments would like to and know how to comprehensively use AI across many services, which the consumers would love (though some would be confused by it). But legal, marketing, and some guy in a department called "Annex B?" are preventing it. So then the people in those departments get bored and go somewhere else and their perspectives and skills are lost.
I know from experience that search in user reviews is very hard. Unless you really understand the review (which was tried via sentiment analysis) you cannot rank results well. But now with the new LLM models I think it would work better.