Thanks for all the thoughtful contributions! Certainly there are some perspectives on this that I hadn't contemplated. If you can pardon the pun, I guess the "bottom line" is they do it because it makes them money as
Grailstair points out,

I guess I need to explain that my question "why do airlines sell miles?" was essentially a rhetorical one to remind novices, embarking on this fun and rewarding pastime, to bear the profit motive in mind before parting with their hard earned cash.