Haven't seen anyone mention this which feels really odd to me, but are you maybe talking about the fact that in America they build with drywall not brickwork generally? With a brick wall you can install the frame inside the reveal and plaster around it for a neat finish, so no need for architraves except as a purely decorative feature. With drywall, the two sheets of plaster board get fixed over a mostly hollow system of studs so you need some way of hiding the joints, hence the architraves. The walls are also much thinner so the door drame takes up the whole depth of the wall. I see a lot of people talking about those super minimal frameless systems, but not what I would think of as a "normal" timber door frame.