I think the primary problem is the inability to measure performance, and thus have empirical values for what someone actually brings to the table. In sectors where measurement is comparatively easier, such as in sports, you can have players earning orders of magnitude more than their teammates for doing arguably the same job. Sports have their issues too, but I doubt you'll hear Yaya Sanogo at Arsenal FC in London complain about all this money that his teammate Alexis Sanchez is being paid yet both are attacking players. Geoffrey Colvin of Fortune magazine once wrote about this some years back, you could probably find it with a bit of google.