From your own calculations, owning a car is cheaper. It's really lame that zipcar costs so much considering that privately owned cars have a utilization rate of less than 10% as you say. We need a more realistic calculation though. Zipcar also charges you $9/hr for every hr you are using their car but not driving. This is the real problem. You spend most of your time with your car parked somewhere as you do something so the true cost of zipcar is much more. The way you do your calculations, a privately owned car is essentially free while not moving. Also, a privately owned car has very large purchase and maintenance costs too of course.
As others have said, people in the first world have money to burn compared to people in the 3rd world so that is why we all own cars.
However, your point about the economic inefficiency of cars is true. When self driving cars come, I expect a revolution in car usage. A self driving car mostly solves the problem of carrying costs while parked which is the problem which makes zipcar so expensive. It will also drive down costs even more by allowing automatic, dynamically scheduled car sharing.