The article provides the reason --- Waymos don't drive in a consistent, predictable manner that human drivers are accustomed to.

They are like timid, inexperienced drivers. They do things like hit the brakes or stop for no apparent reason --- just because they get confused --- which is not all that uncommon.

From a human perspective, driving is about more than what is going on at the moment. It is also about projecting what *may* happen in the near future. And Waymos can't do this very well.