freeCodeCamp is geared toward people who are completely new to programming, so a single giant project would be intimidating to newcomers, to say nothing of unmanageable what with the tens of thousands of active participants. However, there are a few large projects for graduates of the FCC program to work on, under the umbrella of Open Source for Good
FCC is moving away from isolated lessons and toward a project-centered curriculum (though still individual projects, not collaborative). See this forum post for more details: