1. The NTSB and FAA both found repeatedly, as have pretty much every other study - academic or otherwise - that people _cannot_ focus on watching a task without actually being involved in or doing the task. Any "self driving" model that requires the non-operator to be engaged in driving is simply not humanly possible. So the only reason for that stated rule is the manufacturers know these systems are unsafe, but nonetheless want to make themselves not be liable for their faulty products.
2. The only other "vehicle completely controls itself" system in practice is aircraft flight, where the pilots are necessarily engaged in all critical times, and for the remainder of the time all the systems either give sufficient warning before anything goes wrong (e.g. when autopilot disengages on an aircraft you have a substantial amount of time before impact - not the <=2s warning Tesla gives people), or the aircraft systems have specific extremely heavily trained signals for which there is only a single action and no immediate requirement to regain situational awareness (stall, acas, tcas, etc). The time these "self driving" systems provide to regain situational awareness and then take appropriate action is less than what we expect of trained pilots, yet it's somehow acceptable for random drivers.
The current self driving "you are expected to be in charge of the vehicle at all times" is entirely liability shifting: manufacturers are knowingly selling unsafe products, especially Tesla, they are lying about the capabilities of those products, and then they are saying it is the drivers fault when they fail.
What manufacturers are doing is no different from a manufacturer selling a car with an ABS system that fails, and saying "the driver is responsible for identifying ABS has failed and pumping the brakes if it does, if they fail to do so we do not accept liability for the ABS failing"