I'm going to be brutally honest. I find them incredibly lazy and it bothers me that so much of YouTube is content based on things that other people have created. I realize that criticism, re-imagining, fan art, fan discussion, reactions, essays, etc. are an art form in themselves, but I see so much content that is simply people showing other people's work and adding the bare minimum to avoid copyright strikes that it saddens me.
Even worse is an observation that many reaction videos don't have much in the way of original content beyond a square in which you can see the person "reacting" by watching the video, then remembering that they are supposed to be doing something, so they make a big face or shout or laugh a little too hard, and it's not that interesting or fun. To me, of course. I can't speak for anyone else.
Finding truly original content on YouTube sometimes feels like searching for treasure at the bottom of the ocean.