First off, it's a hard coded area, not "route". They're not buses. A limited area is part of the definition of Level 4. What you're thinking of is Level 5, which is being able to handle any area and situation.

There are also reasons beyond capability for the cars being limited to an area. One is legislative, they literally are not allowed to offer service outside a given area. Another is maintenance, the cars need to be within range for their team in case of emergencies or accident. The cars have been tested in many cities outside those two cities, but offering a user-facing service has a lot more barriers.