I have to agree that management is difficult. It's a challenging task to coordinate the work of individual contributors who aren't paid or committed to schedules, etc.
But all this is saying is that there are very few individuals who are willing to work on managing open source projects. Or at least they don't get in contact with the projects they could help with.
Open source projects need more contributors than just programmers, but how to achieve that is a hard question. Programmers don't make good artists, UI designers or managers, but finding individuals who would like to contribute is an issue.
Finding artists is much easier for open source games that based off known titles because there is people who like original game or already participate modding community. Many open source games even started this way (like The Dark Mod).
Personally I'm participate development of VCMI (FOSS engine for Heroes of Might & Magic 3) and there is plenty of artists that contributed to modding community in past and totally okay with contributing their work under CC licenses. Sure that for more popular titles it's may be even easier.
But I agree it's really tricky to find contributors who going to work on brand new game or professional software like GIMP.
Maybe everyone would benefit from a centralized solution where open-source projects submit requests for designing something and designers can have a go at it. This way designers will be able to quickly browse and find the projects most appealing to them / suited to their skills, are able to build a portfolio, and will be able to avoid awkward conversations with uninterested developers. I guess having created parts of a real game or program is worth more than the thousandth redesign of a popular web page, and it actually benefits someone.
But all this is saying is that there are very few individuals who are willing to work on managing open source projects. Or at least they don't get in contact with the projects they could help with.
Open source projects need more contributors than just programmers, but how to achieve that is a hard question. Programmers don't make good artists, UI designers or managers, but finding individuals who would like to contribute is an issue.