I think it's more about detecting changes to the world. You need boots on the ground, so to speak, to see that new speed limit sign or the new lane paint. The Waymo vehicle can no doubt react to changes in the world when it encounters them, relaying them back to the mothership, but it's better to know about them in advance.
It'll shock you to know that you can simply get this from governments, some even provide this in API form
I'd be surprised if this is a thing outside the biggest US (and European, for that matter) cities, judging from Google StreetView there are lots of streets in US cities/towns with almost no paint lines at all.
Then there's roads with just the center line markers with no road should markings.
Then there's a whole class of roads of lines over "demarked" old lines that weren't demarked well, or lines fading that should've been painted a long time ago.
I'm surprised you've never seen a non-perfect road?
But sure, it would be rare to have a large road or street without markings. But most roads aren't large. Most travelled kilometers happen on large roads, but that is not the same thing as most roads. And many individual journeys would involve at least a little bit of small roads at the beginning, end or both.
And of course, if they are covered with snow and ice during the winter you can't see the markings anyway.
A human doesn’t need to be shown every single road that exists in order to drive.
One example, from the end of the journey: knowing in advance where the actual entrance to the business is, or the specific curb cut that leads to the residence, makes it easier and far less error prone to decide exactly where the journey should end. Even humans have a hard time figuring out the right access point for a business or residence. This is a job for an offline process, fed by as many data sources as possible.