But the forks often DO come back. At my employer, we open source our work on FreeBSD.

In addition to being full participants in the community, contributing our work back gives us a lot of other advantages. These include a wider audience for code review which improves code quality, much, much, much easier integration of fixes and features from upstream, the ability to collaborate with people from other employers, the ability hire contractors who are familiar with our code, etc.

Note that I don't speak for my employer.