> They can literally leave school and already be professionally competent.
Do you have evidence for that?
No training makes someone professionally competent, only being a professional does. Even people who go through intense years of law school or medical school, studying directly and interactively with world-class scholars and peers, in actual labs and courtrooms, are not professionally competent lawyers or doctors. People with PhDs are not professionally competent professors.
But focusing on training: I don't just accept any training when hiring. What evidence do you have that videos on YouTube provide a similarly high level of training? The Internet is filled with nonsense, misinformation, and disinformation, and it's clear that people don't make the effort to discrminate between that and truth, and between just any truth and the best knowledge and training. I'm certainly not giving someone or YouTube the benefit of the doubt.
A strong counter-signal is someone saying they learned from "YouTube", which is like saying they learned from "a book", and demonstrates a lack of discrimination or even an awareness of it - here is someone who lacks even the awareness that it's problematic.
And learning from a video or a book has never been a high standard of training; it's been a pejorative - 'they learned to be a doctor from a book'. It's just not the same at all as interactive learning with an expert (a professor), with peers, with a diverse cirriculum, resources such as labs, an institution geared toward creating effective learning programs. Again, it shows a lack of discrimination and judgment, and of even enough thought and skepticism to be aware of the issue.
People like YouTube because it's convenient, and because it's trendy they can get away with not questioning it (and dismissing these issues). I hire people who are committed to being the best, who think deeply, skeptically, and insightfully. I don't want to hire those who make serious decisions - such as their career preparation - based on convenience and who turn over analysis and decisions to the Internet crowd-think.
Anecdotally, I've had people assert that to me how informative certain YouTube videos are and show me them. When I take the time to research it, most are nonsense - I just lack expertise in the field and couldn't tell by watching (research establishes that people are very poor at that, and I'd guess that the obviously false ones don't get views). I have sought and used some videos from specific sources, but then I'm not learning from YouTube, I'm learning from that person. YouTube as nothing to do with it; the medium has nothing to do with it; if they wrote a book or article I'd have used that.
One way of thinking about the problem: Why is learning from YouTube any better than learning from a book before YouTube?