Actually, I think part of it is that when a person actually IS the President, instead of just wanting the job, they get the full briefings from the CIA, the NSA, the Joint Chiefs of Staff, and all those civilian organizations and then really, really understand that what they say gets people killed, or not killed, or in jail or not in jail or economically destroyed, or not, and so forth.  The interim briefings they get as the candidates aren't the real meal deal.

Then, after they are elected, what they said which got them elected is less important and they do their best with the real world facts they have to deal with.  

I think that there is a reason that a sitting President's hair always seems to turn grey before they are done with their second term.

But, as I said, I'm an idealist and don't really believe that the sitting President is actually in league with Satan.  Stupid, ignorant, poorly advised, clueless, all those things I believe about several previous and current Presidents