Books are enough for understanding anything, you'd just need good from-the-ground-up textbooks and probably months or years to read them.

In practice, this isn't true. Some people really do have trouble learning from books. Simply reading the book aloud to them is enough to overcome the block.

I don't know where the problem originates, however. It seems strange to chalk it up to lack of motivation or stupidity, given the people I know.

In other words, books contain all of the knowledge necessary to understand anything but not everyone can pick up the understanding itself from a book. Why, I don't know.