Look at how nicely all the A-based sizes fit together! How did we get the size of US Letter paper? Roy van Rijn explains:
The origin of this size is very vague and lost in history. Most origins say (from Wikipedia):
The 11″ length of the standard paper being about a quarter of “the average maximum stretch of an experienced vatman’s arms.”
Of course, the US legal system is still getting over the change from US legal–size paper to US letter. If anyone tried using A4, they would probably wind up in jail for contempt.