Cognitive dissonance. Knowing stuff is (generally) smart. Not knowing stuff is dumb. If someone's superior knowledge in a field makes you feel dumb, you assage your self image by saying you're not dumb, you're just not a try-hard nerd.

Not being able to appreciate that people are better than you in some fields (or maybe even most fields) might be dumb, but that could be part of a self-reinforcing cycle of not learning or trying because you might not be successful.