I've lived in Washington for 10 years and I have found several roads I am familiar with that have changed due to landslides in that time. I would guess at least 10 roads in the state have washouts that change the course of the road every year. In the wilderness they often aren't repaired and become footpaths.
I can only speculate but I'd guess the issue is that the trees and mountains casue a lot of multipathing, theres no wifi stations or cell towers at all to cross-check, and the traffic volume is so low, you can't really fall back to statistical methods. That doesn't even get into the fact that there's often a labrynthe of private roads that all technically have the same name, typically given by rhe nearest large creek or stream. Google seems to have a REALLY hard time differentiating the gated private driveways from the more "arterial" ungated sections of these roads.