They aren't anymore. That was a 2006-07 fad that wore off after a long period of time when the internet exploited it. Now the only people who enjoy those things are young children, parents, and old people.
People like to watch creatures inferior to themselves do silly things (which is why slapstick comedy is so popular), and cats are cute into the bargain, so people watch videos of cats doing silly things.
Because people have nothing more productive to do than watch their cat until it does something what they assume comical and others think it's funny cause they are using the internet and all of its potential to look up cat videos.