Industrial Organization Puzzle

In IO theory we teach our students that price discrimination requires monopoly power and monopoly power allows the firm to make above-normal profits. So why don't the industries that practice a lot of price discrimination seem especially profitable? Airlines, movie theatres, universities - all classic examples of users of price discrimination yet none seem especially profitable. There are examples of profitable industries that use price discrimination, pharmaceuticals for example, but there are also profitable firms that don't use a lot of price discrimination. If we graphed use of price discrimination against profits would we find a positive slope across the economy as a whole? I doubt it, yet this is what the theory would seem to predict. Send me your thoughts.