I think because a proprietary IP owner can allocate some budget to sue the infringers. A community of OSS contributors may lack the money, the will or the know-how to do that.

They will also face a much harder task when explaining their case to a judge. The contributors to the open-source chess engine Stockfish needed a lot of time and energy to convince a German court that it was illegal for the commercial engine Houdini to copy their algorithms.