in reply to Why do People like larry or David Cross write free language and distribue

A few reasons:

Altruism: They might find it rewarding to share.

Tit for Tat: They might hope that if they share, others share with them

Pride: They might think they get known for their work, their code could be used by many people

See it this way: If you sit around a fire and are the only one with a pack of beer, would you share? Now if you share source code, the situation is even better: By sharing you are not losing anything, you still have your copy of the code.