There's supply and demand, and then there's also principal agent problems. In practice, shareholder capitalism is a lie: companies are run for the benefits of upper management.
It's not only rank-and-file employees that have little leverage, average shareholders aren't much better off.
The government can try and make sure to keep barriers to entry low for competitors, and barriers to switching to competitors for employees. Competition keeps companies in line, even when it's executives running the show.
(You see some more examples of stories like the Ballmer resignation, when you look at the "Effects of untimely CEO death", eg http://www.cnbc.com/2016/04/28/the-effects-of-a-ceos-death-o...)
It's not only rank-and-file employees that have little leverage, average shareholders aren't much better off.
The government can try and make sure to keep barriers to entry low for competitors, and barriers to switching to competitors for employees. Competition keeps companies in line, even when it's executives running the show.