Why do we expect to have good managers when we promote based on technical competency rather than leadership ability? Sure, a certain basic level of technical competency is needed, but after that management and development require different skills.
I consider myself highly technically competent, but if you asked me to lead a team, delegate, etc., I would crash and burn. I just don't have the interpersonal and communication skills to set expectations.
Could I learn those skills? Oh, certainly. But why? The work doesn't interest me. The more I'd go up the management chain, the more I'd get disconnected from the technical work that I actually find interesting, and my day is spent in bullshit meetings where we'd rehash the same bullshit plans on a weekly basis.