Explain how a car with a human driver will be cheaper or have price parity with a car that requires no paid human driver. The driver is the most expensive part of every ride. And that's before tip.
But the main point is: if there is no need to compete in price, companies won't do it. There is no other company offering driverless rides and even if Google were to have huge margins on every ride, they wouldn't lower prices.
Happens all the time that companies literally just choose to not compete in price because people don't have options or just have accepted a specific price level.
That cost will almost certainly come down, but at the moment it is cheaper to just pay a human to drive a car with $0 of AV hardware.