There is no competitive pressure
within a company. CEO pay is usually an insignificant fraction of a company's revenue; there's enough inefficiencies both within the system and within the market to allow this to exist, even if the actual product of a company is a commodity.
The other part of my comment also mentioned that companies do everything they can to avoid being commoditized: that's another source of inefficiency/surplus revenue. The market is in flux; commoditizing takes time.