I used to feel the same way, namely that the tunnel driving would be the signal that unsupervised driving was ready to go. But now that I see their geofenced robotaxis in operation I sort of get it now. Those things do well at driving autonomously, but there are edge cases where the car's safety response is to essentially shut down and wait for human intervention rather than risk uncertainty. That's not something you want happening when you have a single lane of traffic with no passing, leading to a situation where every car in the chain is now forced to deal with the situation in unison. Humans can figure that out pretty quickly, but training the cars to do it will take time. I do expect autonomous tunnel rides any day now, but I can see why they weren't the first use case for autonomous vehicles.