Easy - he lied.
Trump didn’t’ hire the "best" people in the first place. He hired a bunch of wealthy, upper class individuals, assuming they'd all blindly obey him (‘birds of a feather’ and all). I’m sure he also lured many of them in by promising them that if they took the position, it would “vastly increase their wealth” (one way or another). Just because one has wealth or is a CEO of a large corporation doesn’t mean they have the ability to do any “high-ranking” job in the world. It also doesn’t mean they’ll blindly follow anyone else (you don’t get wealthy by listening to idiots). Yet, if anyone dares to disagree with Trump or if they make him look bad (be it true or not), it's "you're fired".
Ever notice how many of Trump’s “firings” aren’t done face to face? Is he afraid of the people he hired? Doesn’t he have balls to fire them face to face? Having to learn about your firing via a news report or Twitter is kind of like having your boyfriend/girlfriend of 7 years breakup with you via text message. Of course, by firing anyone who refuses to follow/obey blindly, Trump thinks he’s is sending a message to those who are left – “Do what I say or you’re next (please don’t hit me)”