There are two explanations, both of them simple as pie:
- As I have said before, people need stuff to consume, that is why all sorts of people buy all kinds of newspapers, in an attempt to keep them afloat. All of them remember the time the internet was dry of information: no Wikipedia, no news, no pictures. People would get bored, web usage will decrease and everybody loses money. That is why Facebook hosts videos and pictures and people share news and information. That is also why Facebook ignores 90% of copyright reports and allows pages to grow on stolen content from Youtube and 9Gag and Reddit.
- Google. Why send traffic to sites displaying ads from other outlets when it can keep the revenue from those ads? Facebook is attacking Google on Youtube and Ads front, but, as I said above, doing the same poor job at client service as Google, killing its helpers slowly.