Some of them are, sure, but one of the skills necessary to effective management is understanding of the work being managed (especially the work at the immediate subordinate level and, if it exists, the next level down.) At the executive level, that probably doesn’t require more than very casual awareness of any of the line work, but at the level of first and second line managers it absolutely does, and its usually a lot easier to find a worker with the right talent to learn the additional skills of management than a “generic manager” who can OJT learn the domain.

I’ve spent most of my adult life in and around tech orgs, and the next even half-competent line- or second-level manager that didn’t start out as an IC I meet will be the first.