...Only the airlines do it that way, and probably because they save money doing it that way.
The airline mileage-based award programs are legacies of the original program inceptions in 1981 (AAdvantage being the first launched on May 1, followed one week later by United Mileage Plus - and I am a charter member of MP). Originally, the only way to get miles was to fly miles. It was a simple concept. Those were truly programs to reward frequent fliers. Some airlines have tried to apply multipliers to reduce the miles earned with low fares, but by and large the airline frequent flier system using miles flown is just too entrenched and the program members are too passionate about their miles to permit conversion to any other currency: and, make no mistake about it, FF miles are currency.
Besides, isn't it far more impressive to say you have an account with half a million miles than half a million points? What's a point? A point is made-up. It's intangible. Half million points? Big deal. But a mile is real. You can measure it. It's 5280 feet. There are 2,521 of them between Philadelphia and San Francisco. There are 239,000 of them between the earth and the moon (on average). Miles just seem so much more impressive than points, even if some of those "miles" were earned by sending your wife flowers on your anniversary.