They're "mobile" because a fixed facility would become a "homeless magnet", neighbors would object, and real estate interests wouldn't like it. Santa Monica has something called SHWASHLOCK; free showers, washing machines, and lockers for homeless people. It only serves about 125 people, though. SF has about 13,000 homeless. (In comparison, about 75,000 gays.)

SF has become more homeless-hostile as the city has become more prosperous.