Lots of legal documents in the US are required to be filed in 216mm × 356mm (sometimes 330mm, both are considered standard and fit where necessary), everything from court orders to congressional acts and those stupid executive orders the orange fascist loves to sign about anything that he manages to think about for more then 2 minutes.

But the bigger problem is that the size is also standardized on the storage and processing side, so switching sizes means we would have to change all the governmental filing cabinets, folders, folios, binders, etc too, which is slightly less likely than us finally adopting the metric system.