the question sort of misunderstands the purpose of mileage programs

The reason they were launched was to get flyers to stay loyal to a particular airline, no matter how much or how little they spend.

The airlines wanted the loyalty of someone who takes two trips a year to visit grandma, as well as the businessman who flies numerous trips.

In some ways the importance of retaining the little guy has increased with the advent of Southwest and other discount airlines. The loyalty programs can be an alternative to discounting fares, a point of difference.

But that said, the airlines do offer the tiers to reward heavy flyers and the credit card companies--which are more interested in the money than whether planes get filled--are increasingly offering some money based programs (e.g. United "choices" from Chase