The only reason Trump is president is because of his fame or “persona”
For years he cultivated himself as a brash, say what you want to get it, bullying businessman who gets it done regardless of what it takes. In the Art of the deal he (or his ghost writer) brags about how he enjoys lying hype up the value of what he is selling . He doesnt believe in Truth the way most of us do. Sure Politicians lie, but Trump’s persona is built on repeating a lie often enoughand brazenly enough that people arent so sure anymore.
He ushered in the era of “alternative facts” and “fake news” as Trump explained ““You know why I do it? I do it to discredit you all and demean you all so that when you write negative stories about me, no one will believe you.” It isnt about truth if a story is negative he doesn’t want you to believe it it is “fake”
That is who he is, that isnt something that happened 20-30 years ago that is his very being.
To a lesser extent there is his objectifying women (including his own daughter on a radio show that isn’t worthy of mention) , demonizing those who fall from favor . Again these arent specific events from years ago that is who he is. People don’t change at age 70 .
But the main reason (in my view) is the first.