Because of the popularity and the culture around it, contributing on github gained a cool status. Besides that, hiring companies accept and often ask for github profiles, so contributors can justify the time spent writing code for another project with having a better looking github profile. It's a lot to due with getting recognition for your work and gaining visibility.
Of course there are other reasons why people contribute with code, but these are extras. I know I would probably be tempted to include a new feature in tincr instead of creating a new extension from scratch.