Since it's an endless hub for entertainment.  People who claim that it's just full of shit posts are kind of dumb and are generalizing millions of users.  Some subs are kind of dumb and full of 12 year olds like /r/funny & /r/adviceAnimals and indeed most of the default subs but there are a lot of quality subs.  The only real alternative to Reddit is 4chan, but 4chan is far less organized and "civil".