I worked abroad for 12 years, 10 even in China where dual sims are even more common. Even my Chinese colleagues would often use an iPhone and just switch the SIM in the airplane, it isn’t that hard. Dual sims are actually meant for (a) people with a personal and business SIM or (b) people cheating on their wife (so an official SIM and a SIM for the xiaosan(s)). Both very niche even in China.
Dual SIM simply isn’t really a feature that would drive phone sales in the states.
Niche maybe, but certainly useful enough to be included.