Swordless Samurai's Journal: Why Bother Learn OpenGL?

I know everybody says you should learn OpenGL because it works on all platforms, but honestly the coding method's are not the same for each platform. To code OpenGL in linux and in windows, you need to know almost 2 different declarations for everything, so my question is if you are going to learn a graphics library, why not just learn OpenGL if you are going to linux only, or DirectX for windows only?