Compare it to NYC where you commonly see beat cops walking the street and patrolling every other block. And more importantly, giving out tickets with an apparent belief that people should get penalized for committing minor offenses. SF Police is just not present or interested in doing this, or resourced to do so. They can't even stop people from breaking into cars, and you think they're going to be ticketing people for driving in a partial pedestrian zone?
I chalk it up to incompetent city government (which frankly right now, is actually hostile to proper policing -- with a DA who has openly said such, and a public who mistakenly thinks this is good to assuage their guilt), a lack of strong police culture, and the incredible cost of hiring people in California, that keep us from having the right amount of police. Honestly, this cost (driven by housing by the way) leads to our crime rate also.