It could just be different interests. The kind of person who makes a game engine is a technical optimization-focused tech-focused person, sort of like a mechanic. In order to make a game, you have to deal with softer concepts like "is this fun" which is more like a designer/artist. Game studios need to bring these people together, but in the FOSS world the mechanics are happy to spend their time building an engine that runs beautifully without concerning themselves with the art side of things.