It’s just posturing to project strength and appeal to his base who want a strongman dictator. He did actually talk about buying Greenland during his first term as well. It’s probably just because it’s somewhat near the US and could conceivably be acquired without starting a world war. Not that I think it will happen.
I know we’re all supposed to be dunking on this for being “cringe” or whatever but honestly what’s the harm? It’s an object and if it brings people joy that’s their business not mine.