For the same reasons that fast food is popular. It's basically dumbed down mindless consumption. I used to be on reddit because I could talk to like-minded people about interesting topics, but the vast majority of people on the internet just want to be entertained and with the support of the admins, they ended up taking over the site. Sure, there are some niche communities where you can have valuable discussions but their time is limited. It's essentially an artificially accelerated Eternal September.