You're operating on a completely archaic understanding of AI. Modern systems can and do adapt to situations on-the-fly. What's more, they can adapt on aggregate - the experiences gathered by a single car can be shared to all cars. Very quickly, every car in the fleet will have billions of hours of cumulative driving experience.
The overwhelming majority of motor vehicle accidents aren't weird and unpredictable edge cases. They're tragic but mundane events that result from a handful of root causes - inattention, excess speed, poor judgement and unnecessary risk-taking. "Driver/rider failed to look properly" is the key contributory factor in nearly half of road traffic accidents. Computers utterly dominate humans in this respect. For every accident caused by some bizarre and unpredictable set of circumstances, there are thousands caused by someone doing something obviously stupid.
A computer can be programmed to be ultra-cautious in difficult situations. A computer can maintain 100% vigilance 100% of the time. Humans can't. Self-driving cars will undoubtedly fail in new and unexpected ways, but it's abundantly clear that they'll fail much less often than humans.