This is a security / immigration policy issue in the US, not an airport layout matter (although the former has now largely dictated the latter).

Airports such as MIA were specifically designed to handle international transit and many gates at IAD could do so as well, but that is irrelevant and has been since the mid-1990's.

Several stations now have international-to-international (ITI) baggage service which largely eliminates Customs for checked bags, but all passengers arriving in the US are processed at a POE (which might include Pre-Clearance).