Thing is greyed out options and tooltips are both set by the application developers and windows has no control over it. I think that if windows took away the creative control of the developers by adding a pre-set greyed out tooltip people would revolt and not use the enable/disable option but instead use another method.

New users find a lot of things frustrating, changing this now would cause more problems that leaving it in place regardless of how good bad it is. Computers are in general supposed to be a metaphor for the real world so if someone can think of a real world metaphor for greyed out options please tell. Smiley