Comment by nostrademons

You might have the causality reversed. Another model might be that the electorate naturally divides into tribes, for a similar reason that competitive sports exist: people want to have a team to root for. And then media needs to adapt their message to make it seems like they're on the same "team" as the viewers/readers, because that's the only way they get clicks. So you may have the same parent media company running different spins on different brands to get left or right voters, but their only true incentive is to make the most money by getting the most clicks.

Arguably, the reason that the pre-Internet media oligopoly was more centrist was simply because it didn't face competition. If you were NBC and ran a moderate story that didn't quite please hard-core conservatives or leftists, they could...go to ABC and get the same story? But if you do that now, the MAGA types will go read Infowars instead, the leftists will go read Wonkette, and you'll be left with no viewers and no money.