I'm guessing it's because a lot of people are still so guarded about sharing that kind of information. As far as I'm aware, it is illegal just about anywhere to limit employees sharing their salary information. Yet it's still not common practice to do so. If individuals aren't really pushing to make it common practice, and aren't willing to be transparent themselves, it won't likely ever really gain traction. Too many people, and companies, would rather wheel and deal to save a buck or deal than deal with the core problems like employee retention and morale.