We do have self-driving cars we just don't have any we think are good enough, well-tested, and adaptable to mixed-use driving and our various regulatory and safety and cultural needs. You can't get regulatory approval for a car that learns to drive by guided trial and error in the wild -- which is the most direct way of training such a thing. Feedback on what you shouldn't drive over also isn't as immediate and unambiguous, compared to whether gravity will topple it over.