They have these already, they are called buses.
I'd still rather be in a private car and just pull over whenever I want to use a bathroom, or get food, etc. I wouldn't feel particularly comfortable sleeping (particularly with my luggage) on a public bus in the US.
I'd also pay extra to have a machine drive me from SF-SEA rather than a human driver, as I'm somewhat antisocial and dislike being around random people like taxi drivers in car-sized spaces for that long.