Licenses are possible, but they're rare. And what can you do with your Prime Directive supplement if ADB doesn't want it? Can you sell it to SJ? No.So there is the possibility of licensing the system.
The licenses that are most helpful are the arrangements where another company produces additional material for the game you want to write for. If you fail to sell your Call of Cthulhu supplement to Pagan Publishing, for example, you can offer it to Chaosium instead. If WotC doesn't want your Dungeons & Dragons material, Kenzer will consider it because they have the license to produce official D&D work.