Ultimately I think this comes down to the paradox of thrift.
You want everything as cheap as possible, which means you want everyone else to not belong to a union. But you want to be paid as much as possible, so you want to belong to a union. Your economic self-interest would be maximized if you were the only unionized person on Earth... but of course that's impossible.
It's not just "corporate greed." I mean that definitely exists and many executives are over-compensated, but compared to other costs like capital, labor, and taxes, that's usually just not that big of a cost center for a large company. That's part of how they get away with it. If CXOs were a significant cost center the shareholders would be less tolerant of those fat compensation packages.