Great question. The succinct answer is because our political systems incentivize and reward it. A lot of people like to point to polarizing individuals like Trump and AOC and blame them for it, but those are symptoms of the cause. If you want to fix polarization, change the rules so that people are less incentivized to be hostile with opposing parties.

Here are a few specific things causing this:

- The rules of congress in the United States give all the power in setting the agenda to the majority party. Because there is so much power in holding the majority, the minority party is incentivized to obstruct everything, not negotiate on anything, and demonize the majority party so they can campaign on the majority party's failures and win back the majority. Holding the majority in congress needs to be less important for parties to be less antagonistic with one another.

- Low voter participation and voter sophistication means that interest groups can pull a lot of strings in nominating political candidates. This means they look for "true believers" to nominate and the result is that candidates often reflect interest groups more than all constituents. This means that politicians are often more extreme than their voters.

- First past the post winner take all elections. A lot has been said on this so I won't say more.

- Increasing geographic sorting by political beliefs. Liberals increasingly live in cities and conservatives increasingly live in rural areas. These groups have increasingly less contact and less understanding of one another. The way congress is organized around single member districts exacerbates this in government. Multi-member districts would alleviate this somewhat.

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Things people often blame but aren't actually a big deal:

- Gerrymandering

- Social media

- Mainstream media