Social media in general loves controvery. So they push radical ideas, as they create conversation, clicks and other interactions, which means more time on the site. That then allows for more ad money.

The truth is most Americans are not that radical, due to just not caring too much about politics. They just want things to keep going as they are.

The other part of it is none Americans on social media. For Europeans for example Biden looks center right for the most part. Then again Europeans have options further left.