And the news networks eat that shit up. They love a boogeyman, because it's good for ratings.
You could imagine a different algorithm that promoted peaceful, thoughtful interactions. But that would have led to the death of Facebook, twitter, news networks, etc.
We may in fact be here due to sheer greed. The media companies have profited by creating discontent in our society rather than content.
You're right, though. Americans actually agree on most things [1]. In that sense, there is really only one "side." Yet the media exploits the small differences that people don't agree upon to create a giant divide.
Anecdote: I firmly believe Trump is going to destroy our democracy, or at least put it to its absolute limits. Yet, I have many friends who voted for Trump. They're great people. We don't ever talk politics, but whenever we talk about economics, or society, we actually agree about most things. If we didn't, we probably wouldn't be friends.
Yet the talking heads on TV would have us believe that democrats and republicans are enemies. And that may very well be a self fulfilling prophecy.
[1] https://apnews.com/article/ap-poll-democracy-rights-freedoms...
[1] >>45191517