The human has unique context. They may work in a niche domain or they talked to people and observed an unsolved problem. Then they express a potential solution via OSS. It's like product sense. Then they share that with others who find it interesting. The code is a great way to encapsulate the idea. It is usually the result of research and back and forth not a single prompt. It would be way harder to think through or build a solution without AI even if they had context.

Because of the convenience. Why should I have to go and spend my time prompting an AI, if someone else has already done that for me. Same thing with food. I know how to cook a chicken risotto, but sometimes I like having someone else do it for me.