There's a wide spectrum of reaction videos, from standard critiques to responses to watch-along videos to montages and edits.
I get what you're saying, but the problem isn't specific to reaction videos; it's a thing about videos which don't actually have any worthwhile content, and I agree that there's a lot of crap on YouTube which uses shady tactics to garner views and ad revenue without having any value to the consumer. Unfortunately, without YouTube becoming a 1984-style police state, that kind of thing cannot be controlled.