That's because the cars are all brand new Jaguars that are impeccably maintained. If Waymo succeeds in becoming a widely used service that is price competitive with Uber, the experience won't be this good forever.
More importantly: the fleet is owned and maintained by Waymo. So they can achieve a higher degree of consistency. It is not the fanciness I value, it is the consistency.
Put another way: I can get a fancy car and a good driver with Uber Black. I would rate my waymo experience slightly above Uber Black.