There's a lot of walking in San Fran, so if you're mobility impaired you either a) stay home, b) hire a car/get a cab/book transport/take a bus, or c) move to some place that isn't San Fran. Odds of you purchasing a
scooter and braving the streets is relatively slim, since they're expensive and not terribly convenient.
On the other hand, if you're considering going on vacation, you might look at all your options and think, "Okay, well, Disney's cheaper than a cruise, and I can rent a scooter to get me around the parks!" Other vacation options, such as going to the beach (hard to get around on sand) or hiking the Rockies (eek), or visiting San Francisco (too much walking), might not look as appealing to you, considering your physical limitations.
For what it's worth, I wondered the same thing on my first trip, ten years ago. It's startling to see so many scooters at first, but then you get used to it and stop noticing.
If my mother-in-law (she who has cancer, diabetes and bionic knees) ever decided to visit Disney, we'd make her rent a scooter!