[belltaco]

It's in part that some people are naturally introverts and thus withdraw into non-social activities like reading, anime, movies, music, gaming, programming etc. It becomes a self fulfilling prophecy because the other kids are out there interacting with others, and thus learning social cues and skills. For example, alcohol famously reduces social inhibitions.

It's not that the natural introverts cannot learn these skills if they apply their mind to it, in fact it's probably easier for them since they're generally smarter.