We also do it to ourselves. Everyone has someone in their life they’d rather mute. But they’re forced to coexist with them. Online is so appealing because you can find communities of like minded individuals. Then forget all about those other opinions you don’t like.
Groups with just things we agree with wouldn’t explain it. What happens is we go into groups of people we agree with and then post memes about the other side that everyone agrees with. We get a caricature of the opposition that is reinforced.
As evidence, just look how much news about Trump gets posted to liberal places. If you don’t want to hear about him that’s the last place you should go.