I write code because I enjoy writing code. Not because I want to be rich, or because I want to change the world, or anything like that. I enjoy writing code, and I like it when people get to benefit from this fact. Why should I charge them lots of money for that?
I also get to benefit from lots of people who feel the same way, and it's a way of giving back to them. I get to use Linux, and Firefox, and .... for free every day, so if I can do something that somehow benefits others then why not?