Isn't that because most of the people that fly multiple times a year aren't the ones paying for the tickets? Instead their company has policies on getting the cheaper flights. And people that fly for a vacation once every few years, they may not have enough experience flying to realize the benefit (or, if they do want a better experience, they go all the way to first class).
I don't if that's true for most air travel, but it's definitely true for my business travel. Corporate policy is the ticket has to be within a reasonable range of price (average price +/- some amount, not sure exactly). This means I can usually select between 2-3 airlines, 2 airports, and plenty of times, but I am always limited to basic economy without prior approval (which has been granted for international travel and if I need to be on a specific flight for some reason).
Back two jobs ago, we had international customers with somewhat high demand to ship our people all across the world on, sometimes on a moment's notice. I had to fly out 4 or 5 times in the span of two years, but some of my colleagues could do more than that in under a year. Somehow, the flights were almost always the cheapest airline available, and the cheapest seats available. The co-workers who flew frequently, including my boss, were all using the accumulating miles to bump themselves a class up.
American tried. American failed. Nobody was willing to pay more.
https://www.nytimes.com/2000/02/04/business/american-air-to-...