Quite some game UI's are done in Flash (check out ScaleForm by Autodesk: http://gameware.autodesk.com/scaleform) so using JavaScript seems plausible as well.
Some older games had the menus fully rendered in openGL. I'm not sure which is better, but I would think our society has more domain experience making menus with flash than with openGL.
exactly, the tooling and skillset for interface design/dev is extremely mature these days, and it probably isn't economical to move people onto custom solutions.