Throw in unboxing videos and the two make up 85% of the content you watch today. It shows how bored you really are without being aware of it. At least reaction videos allow you to sit on the toilet and react along and shit yourself without shitting your pants (British: trousers). Better safe than sorry, sure. But you have to ask yourself if you should maybe take life by the horns and do something more exciting than slurping up cold gruel out of a cardboard bowl while hiding under the bed. Oooo look! I found a nickel! Finders keepers, Mr. Dust Bunny!
Reptilian aliens love reaction videos since they don't have the facial muscles to display visible reactions. The same for zombies. This makes it possible for them to know if they should have their arms raised and make growling noises as they approach you while you are totally absorbed with reaction videos on your phone. Bipolars who actually take their meds and people heavily strung out on Xanax also love reaction videos to remind them of how things used to be.