One possible reason is what you just pointed out: If you have a 20 minute commute and you could trust your robotic driver to take care of it, that is 20 minutes

One possible reason is what you just pointed out: If you have a 20 minute commute and you could trust your robotic driver to take care of it, that is 20 minutes you could be on your smart phone interacting with google advertising. (personally I think the other comments about google doing it mostly because it is a hard, cool, worthwhile challenge are most on the money though)