>>3245498>Supply meets demand
It's also because it's what these guys are only capable of. There is surely plenty of demand for more content depth, but the equilibrium rarely materializes because what these youtubers can offer cannot begin to compete with a chapter from an entry level art book.
Youtube is meant to have low barriers to entry, but this is also their weakness, low cost means low quality output.
Though I was a few times impressed by Kai's sessions with pro photogs and interviews with people like Martin Parr. Instead of playing the expert, which he is not, he consulted actual experts. However, this is more like an exception because these videos are not low cost (huge expenses on company's tab back at digitalrev, extensive networking now that he's freelance).