I think the reason it's so popular to posts content from Tumblr and 4chan on this site is the same reason many people would rather go to a store/restaurant instead of hunting for food themselves. Why search for it in the wild when it's already being presented for you by someone who's done the hunting for you?
That's why I hate it when people complain about seeing that kind of content on here. Who cares if it's from another site? Would the same joke or story be funnier if it originated on Funnyjunk? "Well I don't need to see people's useless comments underneath it." Then don't read them? Seriously, you people are impossible to please.