Because Trump, as weirdly as he may go about it, has made his way through life on his (inherited fortune from his from his crook of a father and) force of personality.

He gets people to do what he wants them to do by projecting an image, a brand, through the way he acts and speaks. Look at The Apprentice. He's playing this role that says (without saying out loud), "Look at me. I'm a straight shooting, no nonsense business leader and my personality and business sense have brought me success, money, fame, luxury, and power. You want that too? You want to be like me? Then do what I say."

Not saying it's authentic or that it should work, but Trump's entire existence is based on this tactic. He's constantly projecting an image, and it's that projection of that image that gets him his way. His force of personality is just as, if not more central to his power as his money (make no mistake, the money is necessary too).

This, in contrast with Musk, who's typically cultivated his following based on ideas and drive. He gets people to go along with him because he's seen as a modern day Edison (and to be fair, the comparison, in some ways, is apt... especially the less flattering ones), a groundbreaker, rules breaker...a visionary.

Simply put, among the people who follow these guys, people follow Musk for what he says (in terms of his big ideas), people follow Trump for the way he acts (in terms of the image he projects). It takes a strong set of blinders to ignore Musk's sharing of his worse ideas and Trump's less than impressive antics, but their respective cults have had plenty of practice in those mental gymnastics.

Thus you're left with the mind boggling (to the rest of us) situation where Musk's followers don't care how he acts, because they are laser focused on his ideas, while Trump supporters couldn't care less about the crazy or incoherent stuff he says, so long as he keeps projecting that macho, confident persona.

So that's how Musk gets away with being "Trump, but more childish": he's not depending on the personality like Trump, so he can act that way without turning his supporters off, because they don't care about that in the first place.

Musk's entire angle is "it's okay to be a humongous asshole if you achieve your goals.

Trump's is, "It's okay to be a humongous asshole, as long as you can sell it as arrogance bred from success."