Because that's been happening forever and had been made far worse by the regressive ideology that calls itself "progressive". It's subjective, of course. To people who like irrational prejudice and authoritarianism, increasing both of those things while corrupting and destroying the ideas of equality, diversity, tolerance, liberalism and freedom is progressive. To me, it's regressive.
No it hasn't, just because you say it has doesn't make it true, like you say that's your subjective judgment it's not actually reality though. Yes there's always been people on "team A or B" but the severity of polarization that we're seeing in today's world is in large part down to people such as Trump, that's what demagogues do, he may not have created the situation but he's most certainly exploited it for his own gains.
Trump used the sociopolitical atmosphere of "either team A or team B" that the "progressives" had done so much to increase. If you're not wholly with them, you're a fascist-alt-right-far-right-nazi! You deserve to be deplatformed, silenced and punched in the face! The brownshirts (Antifa) will be out on the streets to intimidate and attack anyone who isn't wholly obedient!
No. Trump used the discontent in society cause by the 08 financial crisis like so many demagogues had done before him, he used, is using, peoples emotions, prejudice, and ignorance to his own advantage, he like so many demagogues before him is stoking those fires and keeping them going because without that he's go nothing. The reason I'm so confident in saying that is because typically people in his position don't try to make the situation that you rightly or wrongly describe, worse, they don't play on the you're either with us or against us narrative, in fact they typically try to play that down as unlike Trump history has taught them where such actions lead.
The divisiveness that the "progressive" ideology so strongly increased was crucial to Trump's success and he was lucky that Hilary Clinton came out so strongly and so publically in favour of it, promising more of the same and insulting everyone who didn't follow suit. If you're not team A, you're team B and team B are a basket of deplorables!
Trump couldn't have used it if it didn't exist and couldn't have created it when it already existed. Time travel doesn't exist. Though if it did Trump would probably claim to have invented it and know more about it than anyone. He's an arse. But he was (and is) the only available alternative to "progress" towards ever more authoritarianism and irrational prejudice and discrimination.
The fact that you're using the term "
progressive" just goes to show how much you've bought into the narrative of those that seek to divide us, the term is used in the same vain as SJW, woke, and ice crystals that fall from the sky, it's simply a means to keep us fighting amongst ourselves so we don't turn our attention to the real problem.
And the reason it exists, or at least continues to exist, is because people like Trump have exploited it, that's what demagogues do, instead of trying to lessen the divide between people they use it to their advantage.