It's hard to come up with a huge number of cars on the spot once self-driving cars are available. It's much easier to retrofit your already existing cars.
That seems way harder unless they plan on buying a very small number of car models. Every different car model (even the same model between years) has different performance characteristics (do you issue the same orders for "hit the brakes" to a car that needs 100ft to go from 40-0 as a car that needs 50ft?) and physical dimensions which would necessitate different software optimizations and sensor calibrations. Development plus rigorous testing of every configuration means this quickly gets out of hand unless you standardize on 1 or a very small number of models.