I don't know if i'm right but why is it that you only support Windows when using Steam, I would really like to play this game without paying for the OS... ( I am just not a fan of windows.. ) But I am willing to pay for the game thou ;)

There are problems with shaders and other problems making builds for Mac/Linux. Devs don't currently have the manpower (it's 3 person team) to test, debug, fix, compile and maintain any other builds besides Windows.

“Will Raft release on Mac and/or Linux?”
We are first and foremost focusing on Windows. We would like to support more platforms in the future but due to certain parts of the game making porting more difficult we cannot at this time give any promises.