Road capacity has a physical limit. Unlike data you cannot convert cars into light of various wavelengths to pack more into the same physical space. Well you can but that's called public transit. In other words, public transportation has a higher capacity per road/rail area than cars ever will. We don't live in the metaverse after all.
As a result more people using cars (self-driving or not) means you hit the road capacity much more often. That means more congestion and longer trips. Any efficiency you gain by going point-to-point is utterly lost in the increased congestion and traffic.
As the other person noted, European cities are much denser and have significantly less road area than US cities. Thus more cars means significantly less efficiency as you spend more time in traffic.