There are two reasons. One is, the economy is booming. But the other is his opposition. Instead of moving toward the center to pick up votes from those who don't like Trump as a person but are middle of the road or conservative, the Democrats are going ever farther out into the Oort cloud of their own lefty craziness. Their anger and unhinged take on, well, everything also makes those wavering on the other side double down on their choice of Trump, who seems less crazy and unlikable by comparison.
Trump has been very fortunate in his opposition. The more reasonable and inoffensive the various Republicans running against him were, the more "low energy" they appeared. The more disdain and contempt the Democrats heaped on him and his followers, the more followers they created for him. Trump's belligerence and lack of restraint are notable in that he seems not to recognize the boundary lines of proper public behavior, but it works for him. He's not playing four-dimensional chess (plenty of his ideas are just plain bad), but when all of his opponents think they can beat him by summoning up yet more invective and questionable facts, they are playing in his ballpark.
Meanwhile, in the actual actions of his administration, as opposed to the rhetoric, the ordinary business of government seems to be following regular order. And there have been some notable successes that conservatives have long desired: good judges being confirmed; no more apologizing for America; the deep-sixing of the Iran deal; moving our embassy to Jerusalem; dropping out of the Paris Accords. Even the current fracas over border enforcement shows a desire to enforce the law and bring about a more normal and proper flow of immmigration. Plenty of Republicans promised these things and somehow didn't deliver on them after their election. Trump is notable for delivering on key campaign promises, as if that was, in fact, what people were supposed to do who had won an election.
So: Trump will hold the Republicans and the working classes together. Some will hold their nose and vote for the guy who delivered the good things he promised while overlooking the crazy stuff he says and does. Others will look at the increasingly angry and venomous opposition that has learned nothing from the last election and vote against those who express such contempt for them and their values. The bottom line is, unless the economy tanks or we get into a shooting war with a major opponent, I think Trump wins a second term, if he wants it. And I can't see Trump walking away from the fight. So, there you go.