> slapping an overlay of someone looking incredulous over someone else's video is considered

it really isn't, you actually have to provide enough relevant commentary for it to be transformative

it just looks like that because

- not every claim leads to a take down, more common is that the advertisement revenue is redirected to the owner of the original video. That is very very common, especially on YT, but not really visible as viewer.

- there are enough copyright holders which overall tolerate reactions, even if they don't fall under fair use.

- Sometimes people claim it doesn't fall under fair use when they don't like how the reaction is done, but that doesn't mean it wouldn't be ruled fair uses if it came in front of court.

- Sometimes people reacting have explicit permission from the original author to do so, no matter if it counts as fair use or not.

and maybe most relevant here, pretty much all large platforms have a tendency to favor the person claiming the copyright violation over the person which reacted to it. To a point there is is sometimes a big problem if systematically abused with false claims.