One oddly specific reaction channel subgenre is "black rap fan reacts to heavy metal". People seem to love to watch black rap fans starting to like heavy metal for some reason.
No, it's the other way around.
Everyone wants to see someone like a thing they like or dislike something they dislike.
These channels are popular because people want to watch them. You can take a look at the comments and see countless of people telling them what to react to next.