Why doesn't Trump talk about invading Mexico instead ?

I think we can all agree that his rhetoric about Canada and Greenland is ridiculous and that nothing will -likely- happen (and frankly I don't understand why we do as if it will, it makes us look immature. Maybe it's because we think he talks about it too much...)

But regardless of all that, why not just talking about invading Mexico ? -he said they sent too many nasty people (and that led swing state voters to vote for him in 2016) -USA can make a better job at watching the southern border of Mexico -many Mexicans may secretly wish they were American -it would solve the DACA problem for most of its recipients since most of them are Mexican. -they will likely meet less military or economical resistance than Canada So that would make hom look more serious even if he is trolling. Am I missing something ?