Based on the fact that no one is anywhere remotely near being able to do arbitrary pickups and dropoffs of passengers without a driver being present today. And that's totally ignoring any regulatory, legal, etc. realities which I expect will get worked out once the tech is rock solid--which it isn't remotely.
But they are near being able to do exactly that. Uber is testing and training its autonomous-capable cars in Pittsburgh right now, picking up and dropping off passengers on real routes every day. At their current rate of testing and expansion they'll have more than enough training data in the next two years or so. The biggest hurdle is regulatory, but there's so much money and so many lives (remember, human drivers kill people on roads every single day) at stake here that even regulatory approvals are moving quickly. See: Michigan [1].
