Then those aren’t contributors, at least not at first; they’re mentees/students. Projects which can follow that practice are substantially fewer than projects

Then those aren’t contributors, at least not at first; they’re mentees/students. Projects which can follow that practice are substantially fewer than projects who can quickly solicit contributions on e.g. GitHub: requiring mentoring restricts the field to projects whose authors have the time, skills, and inclination to mentor, and restricts potential contributors to people willing to be mentored in new tooling rather than the (much more common) case of people who simply want to provide an improvement to code with minimal friction (and often minimal other involvement with the project, if we’re talking about point fixes for issues faced by the contributing user).