@evan if they don't the project won't be as good, and that's okay, it's a hobby.
It's not the only reason people do things, by far, but it is the only way you can rely on someone to do something. Else, it's up to them.
Generally speaking I'd say if you want to have a great OSS project, you need to put time into it, but that may not be their goal. It may be to publish an idea, to learn a library or programming language, etc.
Maintenance is hard work.