Probably because they're the bottum rung working for someone who is energetic, excited and confident. Essentially, if you don't need to lead anyone, you can have these traits. Or you work for the government.
Better to lack confidence than have too much.
Electrolyte pills solved it (at least, the symptoms). I discovered this by accident, when I had another reason to take electrolytes and then suddenly I could stand up again. Stupid doctors.
And think about it: at the top of that profession (especially surgical specialties), you're basically selling to the uberrich. The field is highly competitive. It's a market predicated on referrals, networks, and especially social proof. Your skill is critical, but skill is a table stake.
Your average, run-of-the-mill, family doctor is a different story. But "baller ass" doctors need to project, well, ballin' assness.
If it's not objective, they're not top.
The doctor who rescued MY life from a disabling illness is humble, because people who are cocky have something to lose if they admit they don't know the answers.
I am specifically talking about top by money earned per year. Not top in terms of best outcomes, or most effective.
Even the ability to project confidence isn't a universal requirement though. Nobody gives a rat's ass what an athlete in a solo sport projects.
Given a choice between someone who projects energy, enthusiasm, and confidence and someone who doesn't the latter person will normally have to demonstrate considerably more competence. In particular low energy and lack of confidence in an expert undercut a perception of competence.