The empire covered something like 20% of the entire worlds landmass. If they spent time in school for every part of it which went on to become something noteworthy, they'd run out of time for any other history at all.
The foundation of the US really isn't as important to the rest of the world as the US thinks it is
We really don't. Our history curriculum is much more concerned with ancient history. As far as I can remember, we spent a little time on the colonisation of the Americas then didn't mention them again until the world wars.