"Normal" people choose their interests, in part, based on their appeal to other people.

Nerds don't necessarily have bad social skills; they usually just prioritize socialization below other things (which clusters with some other non-normal mental traits). Socialization is a side effect of their interests, rather than their interests being a side effect of their socialization. They socialize fine - provided the other person shares their interests, inwhichcase, the socialization advances their interests. They just don't seek out socialization for itself.

This limits their opportunities for socialization, reducing their opportunity for gaining skills in socialization.