Temporary conditions that may come up again and again and again.

A truck with a big load or somebody who just doesn't want to or can't drive fast for whatever reason may end up spending more time driving on the shoulder letting people pass than actually driving on the road -- and the law fully supports this. With the right vehicles or drivers, it's absolutely not a very temporary thing -- it's business as usual.

I've seen some people just spend all their time on the shoulder going 40 mph as everybody passes at 55+ mph, for as long as they were in my sight. And this is considered courteous driving, and is legal as well!

No idea about other states, but Texas seems to be an exception to your claim that "No state allows cars to drive (travel) on the shoulder" -- because Texas does, as long as they're being passed, and if they're going slow and traffic is heavy -- they're always being passed.