If a bad PM/President got elected, in theory there are term limits or other legal methods to remove them from power. Or the people could just vote them out next election.
If a bad monarch got crowned, you’re stuck with them for life. And said monarch probably picked the next one too.
There’s also a very good chance that “someone raised from birth to rule” would be less “trained with the necessary knowledge and mindset” and more “whatever I do I’m getting the crown anyway, so who cares”.