Humans are extremely bad at vigilance when nothing interesting is happening. Lookout is a life critical role on the railways you might be assigned as a track worker where your whole job is to watch for railway trains and alert your co-workers when one is coming, so they retreat to a safe position while it passes. That seems easy, and these are typically close friends, you work with them every day rotating roles, you'd certainly not want them injured or killed - but it turns out it's basically impossible to stay vigilant for more than an hour or two tops. Having insisted that you aren't tired, since you're just stood somewhere watching while your mates are working hard on the track, you nevertheless lose focus and oops, a train passes without your conscious awareness and your colleague dies or has a life-changing injury.

This is awkward for any technologies where we've made it boring but not safe and so the humans must still supervise but we've made their job harder. Waymo understood that this is not a place worth getting to.