In the central cities, Europe's answer to traffic congestion is to reduce the need for cars in general, and reduce the number of cars on the road. Self-driving cars aren't going to do either thing. Streets are already clogged with cars so self-driving ones aren't going to make any positive improvement unless they allow replacement of a personal car with a self-driving car, and that's only going to happen in the long run if self-driving cars are for some reason cheaper than Ubers. Even then, even Ubers/SDC's take up space on the road; there's only so much gain to be had.
It makes sense that SDC development is happening in America, because in America, only car transport is allowed. It has been decided that roadway infrastructure is going to be the only form of infrastructure to get public investment and the only one which will be accommodated by public policy, no matter what. The fact that SDC's aren't really going to move the needle here either, is a lost point because there's nowhere else for investment dollars to go.
Logically, mass transit moves so many more people than cars, that in terms of dollars per person moved, cars are nearly irrelevant by comparison by the raw numbers. A single subway line carries more people than the busiest highway in the world. And cities may have dozens of subway lines. Throwing money after cars, when the streets are already clogged with cars, is just a waste of money, self-driving or not. So European cities are more likely to invest in things like the Grand Paris express, a completely new subway system (which, notably, is electrified and self-driving). By comparison, self-driving cars are just a technology that's seen as a way to increase the number of cars on the road, increase congestion, increase wear and tear on the roads, all for the "benefit" of saving the salary of the drivers, which is an important source of low-wage employment, and it's just not compelling at all.
Having ridden Waymo a lot, and also driven through European cities, I actually think they could navigate through European cities just fine, and also safer than human drivers. It's not a technology limitation IMO. It's just...why. At best, you save the cost of a driver, but even then, you simply can't improve overall transport efficiency vs. taxis/Ubers, which they already have.