As you can see from taginfo:

https://taginfo.openstreetmap.org/tags/building=roof#combinations

Over half of building=roof areas in general now have a layer tag added, and if you consider petrol station roofs, the percentage is likely much higher.

Indeed, the USA is not a thoroughly mapped country, which is why anyone mapping there would do well to look to the countries that are thoroughly mapped, so that the standard of mapping in the USA can be raised towards the level of those countries.