The same things that are preventing small restaurants also stand in the way of that sort of thing. If you could install a machine like a kiosk where people walk up to it, press buttons and insert money and out comes a hot sandwich made to order, people would be renting space to install them all over the place.

But the machine doesn't take up that much space. If the zoning requires you to rent 200 times that much space, the economics go away. So you still have to solve the original problem.