At some point airlines stopped doing the hub-and-spoke model where all flight come into one big airport and then passengers change flights onward to their destination. Now they favor the point-to-point model, which as the name suggests means every destination pair is served by an individual flight.
As to why. It's probably financial but it don't know for sure really.
Do we have anyone here who works in US customs, or whichever agency is in charge of applying these tariffs?
How do you guys even keep track of what to charge when tariffs change daily?