It's because of TIGER! It groups a huge variety of roads together as something like 'minor road, open to the public'. A dirt trail all the way up to a nice highway that they haven't noticed was rebuilt.
Most digital US maps started with it and low traffic areaas are not a priority to update to something more sensible.
There's also enormous variety which may change with the season, weather, washouts, and how recently the road has been repaired across large stretches of the US West, among other areas. Even with a map, the difference between well-graded dirt road and competent high clearance 4WD vehicle and driver needed is often far from obvious.
Furthermore, some visitors not familiar with such areas tend to assume that, if a road is on the map, that means it's fine to take in their passenger car.
Thank you; I honestly did not know that. I've worked in routing previously, but in Europe it seems most road data is much better categorised. It also helps to have less area and fewer roads, of course.
Most digital US maps started with it and low traffic areaas are not a priority to update to something more sensible.