And when the driverless taxis are driving around in big, empty, completely flat, paved plazas, of the sort beloved of town planners' illustrations, then this might sometimes be feasible. (Even though shopping bags, suitcases or anything else one is carrying make short, steep ramps through narrow entrances difficult and dangerous.)
But in the reality of the British street scene, cluttered with parked cars, varying kerbs, bollards, bins, sandwich boards, heaps of building sand, wheel-unfriendly surfaces, etc etc, this is not an adequate solution.
With a human driver there's a lot of, "Oh, you can let me out here because I can see all those obstructions over there mean there's no possible safe place to stop the car, extend the ramp, give me road space at the bottom of the ramp and give me a safe route to a dropped kerb and up onto the correct pavement where I need to be."
Even once out of the car, it's easy to be marooned on a bit of road or pavement without a dropped kerb in sight – and if there is one, someone's parked across it.