You're also forgetting the biggest thing, I think, standing in the way: Intimidation. Perhaps in their minds, they aren't expert developers, and are just learning, and therefore, their contributions to a very large project like Firefox or Linux would be unwelcome--as they probably are a little too scared to start. A self-confidence issue, as it were.

Through the lens of the internet, it certainly feels sometimes like everybody knows everything--you often forget that you're getting thousands of viewpoints rather than just one super programmer doing everything. I can definitely see how this would be intimidating for newer developers.

Cutting your teeth on obscure open source projects is a way to build up your self-confidence, work with others, and see your contributions make a tangible benefit.