I don't think mentoring is necessary for skill building in general wide subjects. I don't need a mentor for writing and neither do you - but I work in a very niche area of British law (among other things) and without having someone more experienced to telephone when the shit hits the fan, no amount of scouring around on YouTube is going to help me. Mentors are necessary for difficult specialists and skilled pursuits.

Also, Youtube isn't a great selector for quality of information. As you've said yourself, it's very easy to set up a course and run a course - the difference between signing up for a quality mentor and signing up for the sort of person who is two sentences ahead of you on the learning curve is important - there's too much smart talking bullshit out there to disregard the concept of mentorship.