Moreover, if you don't like Uber's prices you can spend your money on something else, whereas Skymiles can only be spent within the Skymiles program, so for the program to make sense there needs to be some sort of credible guarantee that you'll actually be able to redeem a certain amount of Skymiles for a certain award.

Otherwise it's like going to one of those arcades where you earn tickets for high scores in skee-ball or pop-a-shot (do those even exist anymore?) and redeem them for some crappy toy, except that there are no prices listed for any of the toys and they won't tell you the prices until you've accumulated a stack of tickets and are ready to redeem them. Who would be dumb enough to fall for that?