Because state DOTs don’t care about other states, it’s (mostly) not within scope for them to do so. That’s the thing. Oregon also uses SR on signs, Oklahoma uses SH. That doesn’t change that this is ambiguous and the ambiguous situation should be avoided. And all three states mentioned have state highways in other states: Oregon and Washington have routes in each other, Washington has one Alaska state highway, Oklahoma has one state highway in Arkansas and vice versa…