Most? You mean all. 240V [0] as been the standard in the US basically since electrification started in the late 19th century. 120V has for all practical purposes never been a thing, it has always been an artifact of split-phase 240V. A deliberate choice to offer two voltages to every consumer.

[0] Okay, technically 240V did not become official until around 1967, but the split-phase design was there from the beginning. They capped it at 240V to stop the creeping up that had been going on in the earlier part of the century. This is why you still have a lot of people (not all of them old enough to have been alive in 1967, oddly enough) that refer to 240V as 220.

In tge early days they did sometimes wire 120 only houses. That was mostly done before WWII