[keeperofdakeys]

While both may be Posix systems, the GUI, mouse and audio apis are much different - so there is still a lot of work you'd need to do. Since both use opengl though, you get rid of half the trouble by porting.

It's also important to remember that many game makers don't make their own engine, and so the engine must support OSX/linux (most classic linux ports have used the unreal engine, or source engine). Unity is quite good in this regard, you can hit a button to export your project to all three different platforms.