For one, it's our male role models who are using computers while our female role models aren't. Children are affected by such gender differences very early. They notice who drives the car when everyone goes out, who cooks dinner, and who works late.

Secondly, there are expectations about it. For example, if a boy takes apart a computer, everyone goes "such a boy", but if a girl does it, people say "hmm, that's odd." (even if it's good surprise, it's still surprise). I know when I was a child I was pushed to do things like ballet and cheerleading and orchestra when what I wanted to do was martial arts and read and become a scientist.