Your news feed is designed from the ground up with powerful artificial intelligence to become an echo chamber, and lots of FB users just don't understand this. They fall for it completely.
I have more liberal friends than conservative friends, and my FB feed literally only shows me anti-Trump pieces (along with ads and other spam), some of which are astoundingly blatant in their bias.
I have to go out of my way to seek out opinions from or articles shared by my conservative friends. Even if I do this frequently, FB still does not incorporate them into my feed. Instead, every single day, it shows me low-quality clickbait anti-Trump articles shared by someone who lived on the same floor as me in a college dorm 7 years ago with whom I shared one conversation in real-life and whom I've never interacted with on FB in any way aside from approving her friend request.