Because he has an ego the size of Jupiter.
This is the entirety of the answer. No other factors play any real role here. The fact is that once you realize what a raging narcissist Donald Trump is, everything he does makes perfect sense. His narcissism alone is an adequate explanation for everything including his run for president, which foreign leaders he likes and dislikes, his bullying tweets, the fact that he touts the vaccine as an accomplishment despite his base hating it, the whole stolen election nonsense, the fact that he'll never disavow extremist groups that likes him, etc.
Every time somebody asks "why is Trump doing this?", think whether his narcissism explains it. The answer will almost always be "yes, it does".
