You do not get very much for contributing to an open source game: A better game, and an entry in the credits. For that, you have to give up most of the monopoly rights granted to you by the copyright laws around the world. That is easier if everyone gets the same deal and nobody gets a better one. An example: You may want to include stuff that is not free software, I would like to include code licensed under the GPL. We cannot both do that because the GPL requires that there are no pieces with more restrictions than the GPL has, so we have to compromise. Requiring very liberal licensing with neither "sharealike" restrictions like CC-by-sa and GPL nor restrictions which would prevent the result from being free software is such a compromise.