Because they manipulate the news feeds spoon-fed to their members.
Although FB claims their “trending” news stories are “an organic or algorithmic representation of user interests and activities,” they actually have a whole team manipulating which stories get fed to the “trending topics” list. As the article makes clear “Managers also instructed them to insert stories that were not trending despite being in the news.”
Now, I have little or no sympathy for anyone who gets their news from Facebook, but this illustrate the normal level of respect FB gives to its members.
Previous reasons NOT to use Facebook.