Board members have a portfolio of firms they are on, so they are less exposed to any single firm than staff. They are also part of the group that might be named CEO if a new one is needed, so it's in their interest to keep compensation high.
The board and the CEO are people who have known each other for years.
On the other hand, I don't think that is the only source of problem. Companies have gotten way too big and the experience of running global conglomerates is simply not common. Add risk aversion to it and the same people, whether succeeding or failing, end up getting hired over and over. It's a lack of supply and lack of competition.