[Tenhundfeld]

That's a fair point, but I think that clean idea can be messy in practice.

For one, engineers rarely manage people before they are managers. They might lead a team or be a product manager, etc., but those have different (albeit overlapping) challenges from managing people.

Another important factor is that in small or fast-growing companies, there's often not a formal promotion to manager. One day, you're the solo dev or maybe the you're senior dev of some secondary system. That system gets unexpected popularity/traction, and your manager starts throwing resumes or new recruits in front of you. And before you know it, you're managing a team, still trying to lead code reviews but also dealing with personnel issues, manage timelines, etc. You weren't really promoted. You were just the person in charge of this thing, and the thing grew underneath you. I'm not saying that's ideal, but it's something I've seen fairly often.