I wonder what this looks like if you break it down by industry and firm size. It seems odd that this sort of thing can persist (and increase) over time since you'd expect competition within industries to drive this down over time.
Perhaps competition drives executives salaries faster than general workers. There is intense competition for those who can have a large impact on the bottom line. In tougher/faster paced competitive environment the demand for qualified/experienced individuals must be intense.
You don't have to scrape out a lot of Walmart's revenues to get a huge salary. You have to scrape out a pretty big chunk of the revenue of a 50 person firm to get a large salary, never mind a huge one.
I think that forces driving the market are often, these same guys who are getting paid.
https://lwp.law.harvard.edu/files/lwp/files/webpage_material...
Executive pay is not set by "the market". Executive pay is set by other executives sitting on each others boards.
How surprising this executive class somehow consistently votes to give their fellow executives a greater and greater share of their corporations' value.