I can believe that someone would have the discipline to sit down with a textbook and teach themselves something. I've done it myself (although I wish I could do it more often). But the problem is that it isn't always obvious what parts are important or why. I found in my first year of university I learned a whole lot more CS than I ever did before, not just because I was being taught at a faster pace but also because I had a better feel for what I should learn on my own.