Because the rest of the world is behind the curve, the US is merely leading the charge as its established political landscape perfectly plays into the filter bubble polarization FB feeds on.
Something that's quite observable in places like Germany: Election campaigns in Germany used to be quite boring, there used to be no such thing as "political attack ads", parties kept their campaigning to topics they stand for, instead of trying to attack other parties or candidates on their particular positions.
At least that's how it used to be for the longest time, but during the last decade the whole tone around German elections became noticeably more hostile, something that directly correlates with the rise in popularity of the AfD.
Said AfD has hired American Harris Media for their campaign strategy, the same company that won Trump his presidency [0]. They not only introduced the wonders of micro-targeting, which already contributed to the Obama presidencies, but also added their American flavor of running political campaigns with these extremely hostile overtones.
The latest highlight of that escalation, during the recent election, has been far-right parties literally calling to "hang the Greens" [1]
Or to give another, often overlooked, example; The US wasn't the only country that saw their capitol stormed in recent times, an attempt was also made in Germany, but there police actually stood their ground [2]
And while most of the big established parties condemned what happened there, the rising far-right ones did nothing like that, they were right there riling people up.
[0] https://www.bloomberg.com/news/articles/2017-09-29/the-germa...
[1] https://www.swissinfo.ch/eng/german-court-orders-removal-of-...
[2] https://www.bbc.com/news/world-europe-53964147