That's why most presidents have had prior experience in government, and why Trump is an object lesson in why you need to know things about the job you want. Apart from the fact that he's a horse's ass, why do you think he has such a terrible time dealing with the Senate? It's at least in part because he doesn't have any idea how the Senate operates. It has arcane procedural rules, like when and how to use the filibuster, and if you don't know something about those rules the Senate will eat your lunch and torpedo your agenda if they feel like it. If you come to the presidency from the Senate you know how it works. You know the rules, the procedures, the power structure and the people.
Or, if you were a state governor before you were the president you know how an executive branch works. You know how to appoint and manage agency heads and how to evaluate legislation. (I think one reason why W was a crappy president is that he had been the governor of Texas, which has a weak governor system, and he wasn't equipped to actually manage anything. And he, too, was a horse's ass).
But if you come from show biz with zero experience or exposure to the operations of government, you will be clueless and are likely to fuck things up even if you don't mean to. You will have to rely on others who will know a lot more than you and might not act in your interests. You could make a major ass of yourself.
What if Ben Carson had been elected? By all accounts he was a brilliant surgeon, but it was obvious from the beginning of his campaign that he had no idea what presidents do. Now he has a cabinet position, which he apparently doesn't understand, either. (Trump was able to get away with being completely ignorant about government throughout his campaign because he threw up so much chaff that a lot of people didn't notice how clueless he was - they were just excited about all that swamp draining he was going to do.)
The point is, great expertise in one line of work does not translate into competence in another. The presidency is an actual job that requires presidents to know and understand actual, specific information. There is little time for OJT. The federal government will be a smoldering slag heap when Trump gets done with it, so more than ever we will need someone who actually knows how to repair the damage. A charismatic tv talk show host will not be that person.
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