Because every night is an A-B test for Tesla — a test that is running on your car. One processor is driving the car (or following the driver’s actions if Autopilot isn’t being used, which is most of the time) with production software while the second processor is running beta software, simulating the drive, and noting discrepancies between the two software versions. Multiply this times a million cars per night.
Whether Autopilot is used or not doesn’t matter: the evolution of the software continues. And it’s finished when the beta software stops improving and the outcome shows the only difference between human and Autopilot driving is that Autopilot does it better. Continue for another month or year or decade just to confirm your results, then announce that full autonomous mode is available. That is exactly where I believe Tesla has been heading for as long as those two-processor cars have been on the road.