There's a lot of suspicion that your presumption isn't accurate. It's a fairly big area of research, though. One alternate hypothesis is a form of "executive capture", that companies nominally owned by shareholders are actually run by more of a clique of executives, who serve on each others' boards and pay each other large salaries. Some mild but inconclusive correlational evidence for that view is that when controlling for other factors (size, industry, etc.), CEOs are paid less at private companies and companies with concentrated ownership, than at companies with diffuse ownership—suggesting that when the owner is in charge and paying attention, they prefer to rein in executive pay.