Arguably that happened because Linux was, in fact, the most generous option: the BSDs effectively refused to get contributors beyond their very small groups, and refused to incorporate patches to support "bad" hardware or nonfree blobs - meanwhile, Linux was built around modules that could contain anything regardless of license, and happily accepted new patches and unknown maintainers. You still see it today: Linux literally built its own distributed VCS (git), while some BSDs are still on the hyper-centralized CVS.

If there isn't an even more generous option is simply because building a mainstream OS is incredibly hard.