IMO the reason is that he is a senile old man lashing out at the world. I really don't think there's a lot to it beyond that. Everyone else goes along with it because they know the administration's entire legitimacy comes from Trump and if/when he's gone the whole house of cards will collapse. That or they have their own focuses (e.g. Stephen Miller) and are happy for Greenland to be a distraction while they get on with their own stuff.