This seems silly -- they roll the service out to individual cities in different regions, one at a time. Why do you think they do that? I'm pretty sure this is exactly that testing that you're referring to.
They can, and I bet they have! But they cannot afford a test track that accurately reproduces every condition exactly as it will be encountered in the real world. At some point, it is judicious to test with real-world conditions, and simulating only gets you so far.