And why should that be?
When I drive my current car, I take responsibility for my driving (by trying not to crash) and the acceptable operation of my car (by maintaining, testing it etc). Then if I cause an accident, it is my fault, I either made a mistake when driving or I neglected to maintain my car and it failed. So, my insurance pays for the damage caused to others.
What you are proposing is to take the risk of somebody else not messing up - I cannot do anything to make sure the AI works correctly, or, in other words, if I maintain my car according to the manufacturers instructions and it still fails, then I should not be at fault.
I do not see why the software of the car should be treated differently than the hardware. If the brake system has a design problem that causes it to fail sometimes without warning, then the manufacturer is responsible for the damage. This is also the case for current car software, so why should the AI be different?