Many people do not like stuff from their game/project being reused in other projects. They want their game to be unique. If you contributed some code under a license that would allow you to reuse it, then that becomes a non-unique object. Why this is a problem is more obvious with art assets than with code, so I'll use that for an example. Say you contributed a ship design to VO. Now, a few years later you make your own game also using that ship, or license that ship to be used by a competitor of VO. Now there is a problem, because two competing games have the same ship. It isn't necessarily a legal problem, assuming all the licensing was worked out correctly, but it is a marketing problem.

Beyond that, it would involve yet more paperwork. Things get annoying when you have a bunch of different licenses covering different chunks of your project. Especially if they aren't easily separated, or if they have a good chance of having bits and pieces spread into other areas over time.

Requiring sole ownership of contributions means that a group of potential contributors are alienated, in order to maintain simpler legalwork and better chances of being able to attract investors and potentially sell the game. It is a compromise that GS has apparently found acceptable. If I were in their shoes I would most likely do the same thing.

It sucks if you happen to be one of those who is alienated, but that is life.

@Pizzasgood guess your right. meh.