The difference is liability. If you're riding a Waymo, you are not at all liable for what the vehicle does. If there is a collision, you don't need to exchange your insurance info or name or anything else (regardless of who is at fault). You are not allowed to be in the drivers seat.

Tesla has chosen to not (yet) assume that liability, and leave that liability to the driver and requires a driver in the drivers seat. But someone in the drivers seat can override the steering wheel accidentally and cause a collision, so they likely will require the drivers seat to be empty to assume liability (or disable all controls, which is only possible on a steer by wire vehicle, and the only such vehicle in the world is Cybertruck).

Tesla has not asked for regulatory approval for level 4 or 5. When they do, it'll be interesting to see how governments react.