Availability. I probably want to use a car the same time as everyone else. Price. If there's enough cars to satisfy demand at all times, the capital costs of the network are high which means rates are high.

Basically, a large operation could have economies of scale (in house mechanics, only two models of car to choose from, own the fuel stations...) but the utilization of the vehicles probably would not change dramatically from the utilization of the private "fleet" we have today.