I've got somewhat mixed feelings on the rule along with reality of the use of the space.
Different areas may experience some differences in use of that space as well, based on environmental conditions, as well as construction types having some impact.
If you have a truss roofing system - especially one that leaves little open spaces in the framing - that space won't get used for much at all, and is not even a point in installing permanent ladder/stairs yet some do anyway.
Most of the US also has enough temperature extreme, hot, cold or both that the space is getting filled with enough insulation that it also is not usable for anything.
There used to be a time where the attic was an additional floor in the home with a permanent stairway, like you see between normally occupied floors not a hideaway/pull down stairway, but most of that type of construction around this area that I have seen was homes built before 1950's when there was little or no insulation being installed in the home.
Today you may find the space over a garage used for storage, but usually the rest of the home has insulation (or at least a lot more then the garage) and is not desired to compromise that insulation for storage purposes.