Interestingly, Paris does have strict height restrictions for buildings. But two big differences are:

* it is easier to find tiny (and thus cheaper) housing in Paris, since the standards of comfort are lower. For people who want a simple shelter and spend most of their time outside, it can frustrating to look for housing in SF.

* public transit is everywhere in Paris and most areas inside the city are safe, whereas in SF people tend to choose from a small subset of neighborhoods. Cars are unnecessary - they generally create more problems than they solve.