Many airlines do.
And if they don't, there are good reasons not to do so. It's about encouraging repeat business, not just selling one first or business class ticket. Many people who fly premium cabins have all the benefits that come with elite status anyways.
If you sold elite status, you also can't control how many elites you have. Airlines can raise mileage requirements, accrual rates, and all sorts of games to control the number of elites. If you just sold it outright, then it just becomes a product, and you actually have to look at the value proposition.
IHG sells Ambassador status for $200, which is also IHG Platinum or Gold. It's also why IHG status is considered widely worthless.
BTW Robert I love your blog. It's spectacular. Didn't know you were on FT.