While there are many, like MANY factors I can put in as to why the country has become as divisive as today, the issue stems from education and the weaponisation of information.
In the old days (When I mean old, I mean 20 plus years ago), you had people that would think for themselves, and think of the next generation, this is all but gone now, that's because majority of American don't get the education they deserved, I mean look at it this way, I was from the US, I lived in Canada, Australia, Sweden, Hong Kong, the common theme for Canada, Australia, Sweden and Hong Kong is Higher Education is either readily accessible or is completely free. I had studied in the Chinese University of Hong Kong for free for 2 years under a "Grant-Loan" scheme, and my wife attended University in Malmö for free; that's just how higher education in Sweden is. I had done 2 degrees in Australia because the government has a low-cost loan that you don't need to pay if you don't earn money, and a subsidy I can apply (called Austudy) that can supplement my income while I study. None of these, all these, are available in the US, which means Americans (53%)are lower than Canada (66%), Australia (55%) or even Russia (62%) in terms of higher education Attainment level, in term of percentage, it may be similar, but when you look at the absolute value, over 150 million American are without degree (That's the entire population in Russia), only ~10 million Canadian, 8 million Australian are without a degree.
Now, people without a degree are not an issue, if we were back in the days, because we didn't have social media, and the internet back in the early 2000s, so people would still try to digest news themselves, and think for themselves. But with social media now, we don't get our news from newspapers, which just report on the news, and have you digest it, we now have pre-digested news from social media platforms. I mean, if you follow Benny Johnson, you will get nothing but right-wing news, or if you follow Medhi Hasan, you will get nothing but left-wing news. So people started to just listen to what these personalities have to say, and believe this is true. And they don't process information themselves anymore, that's created a definite divide because people who follow Benny Johnson are most likely not going to follow Medhi Hasan and vice versa, so you have two complete opposite spectrums, and people fall in line, form groups based on these two spectrums. That's how a population is defined.
But then you might think, wasn't it the same 20 years ago when you had Fox on the right and CNN on the left? Yes, but back then, we didn't have a cultural war happening. I remember what George Carlin said, when you have to define a problem using the word "Wars" like war on drugs, war on terror, war on crime, war on poverty, gender war, and so on, that's benefits nobody but it's just a vessel that takes away discourse and makes money off it. I mean, you don't see it as a problem where it should be, but something you need to win over it, that's what war mean, I know becuase I fought in war myself, there are only one goal, which is to win, which mean you don't actually address the problem, you are just trying to win an argument. This leads to a very dangerous game when we are talking about Politics, especially in the American political structure. The Founding Fathers chose the way Congress, the Senate, and the office of the President work is to make compromise, that's it. It's that simple, because nobody could control all 3 lobbies in absolutely at the same time, you can control the Congress, the Senate and Presidency, but without 2/3 of vote, it won't work in most case, unlike European or Australian system, where you win, then winner takes all, which when this "Warring" culture comes in, instead of focusing on the issue at hand, you just try to win the argument, and your goal is not to further the American agenda, improve people life, no sir, your goal in congress is to not let the other party win, that's why you were there to vote down proposal from the other party, that's because you can't let them win. So when you don't have enough in the house, you need bipartisan support. That's not going anywhere. That's why it takes 10 years to process a bill to modernise roads, ports, and airports.
But at the same time, the problem will not go away when you are playing the political game, which then, as a citizen, you started to hate the other party, Democrats started hating Republican,s and Republicans started hating Democrats, and you keep that fed year after year, you grew further apart, that's how and why America is so divisive now.
And you know it's true because, going back to the Jonathan Pie video I posted, we are coming down to a point where you want to know which political affiliation the shooter who killed Charlie Kirk was, or the shooter who killed Melissa Hortmann was, and be glad it was not "one of yours" (If it sound familiar, that's the same as you hope that the next mass shooter is not Muslim and being called a terrorist, that's the same thing) that is where you know you are divided, because whatever that shooter believe in, be it Republican or Democrats, or Islam or Christianity or Buddism, it has nothing to do with the act itself, the act was carried out by one man, I mean, did we question Lee Harvey Oswald what his political afflilation was after he shot JFK? No, he is an American, and he has done this horrible act, and whether you are from the Left or Right, you shouldn't feel glad, elated, or ashamed for him being or not being one of you. That burden was shared as long as you were an American...