Because developers greatly benefit from participating in open source projects, designers usually don't. That said, developers also make more money than designers do and in a tech centric world, there are more needs for devs than designers. That's why.
For most OSS devs, the benefit is having a more interesting portfolio, especially skills they don't get to use in their day job. Wouldn't this work the same way for designers?
Not unfair at all. The vast majority of developers get paid for their time. It's only in Open Source that you see people volunteering their time with no hope of monetary reward. Even within that tiny minority, most of them are doing it for some form of compensation, even if it's just to get their name attached to something and build a reputation.
If you're developing software for somebody else, they're not paying you, and you're not getting any benefit whatsoever, that's entirely your fault. It's certainly not unfair though.
