"Doing X", and "Managing people doing X", are two pretty distinct skills—and interests. If you find someone really good at doing a thing, why would you pull them out of that position? Odds are they won't be good at it, and they won't enjoy it. There are people who enjoy both, but they're rare.

I'm reminded of Plato's Ship of State metaphor (https://en.wikipedia.org/wiki/Ship_of_State) here whereby the people who come into power are not necessarily the most skilled, but instead those who are most skilled at getting into power. Nobody would disagree that somebody who is both good at management and of a strong technical mind would be the ideal candidate, but usually it is just somebody who is good at getting promoted.

Bertrand Serlet too, who was the original author of OS X's malloc() implementation used during his tenure.