You buy a car because it is there for you. When you want to go it is always where you left it and ready to go. People tend to forget about this: you don't have to reserve your ride ahead of time, you just go when you are ready. To be useful shared cars will be idle most of the time as well: if they are not you cannot be sure one is ready and waiting for you when you decide you want to go. Otherwise you may as well take the bus which is cheaper than a shared car can ever be because more people are on it at a time.

Since there isn't much savings possible in a shared car without losing the whole advantage of a car you may as well buy your own: the ability to leave your golf clubs in it is more than worth the extra costs.