In the case of Greenland, I suspect Miller and Thiel want it for their own purposes and Trump is so far gone they're just telling him something that will stick in his demented brain.

People stop growing once they gain enough power or ability to avoid it. Elon seems to really resonate with the people who don't have friends and live in their parents' basement because both have an adulthood defined by not needing to mature as an adult and not having the internal drive to do it anyways. Nobody can make him feel ashamed for acting like an 18 year old because even if we shame him he can look at his wealth and justify why he's better than us.