The most important aspect of collective improvement (e.g. attending a class, or working together on a project) is that you have forced exposure to other ideas that you would not have thought up or consider yourself. I'm an introvert by nature, but nevertheless I crave working alongside others simply because it works better. I end up having better ideas and being more productive, even if it is emotionally draining as an introvert.
LW has a problem. Openly or covertly, many posts here promote the idea that a rational person ought to be able to self-improve on their own. Some of it comes from Eliezer's refusal to attend college (and Luke dropping out of his bachelors, etc). Some of it comes from our concept of rationality, that all agents can be approximated as perfect utility maximizers with a bunch of nonessential bugs. Some of it is due to our psychological makeup and introversion. Some of it comes from trying to tackle hard problems that aren't well understood anywhere else. And some of it is just the plain old meme of heroism and forging your own way.