Open GL for the GUI?

I have been seeing software that uses open GL for the GUI. (blender for example) So I am asking this: Why not replace gtkmn with Open GL for the GUI? It can run though OS systems with no problem since it relies on the video card. The users don’t have to download anything for the GUI.

So what do you think about this new implement (idea)?

Superanimator, sorry for miss this post.
Please check out what uiomae is right now doing.