In 1992 and 2000 I transited the US at LAX and SFO and we were required to go through immigration.
In 2002 I transited the US on the CX flights HKG-ANC-YYZ and back and at that time the US did not require passengers on those flights to leave the aircraft and reboard.
The majority of flights from US "international" airports are domestic and the US does not have exit immigration controls, so the airside areas of US airports are all "inside the US". For international transit to be enabled some areas would need to be designated "outside the US" and this would only be possible by instituting exit controls or reconfiguring airports in a very complex way.
In the UK the airports LHR LGW and MAN achieve domestic and international mingling by making domestic passengers (who are in the minority) have biometric scans.