SMS/MMS is free in almost all phone plans in the US, and I have absolutely no trouble believing that people would subject themselves to a very bad messaging exp

SMS/MMS is free in almost all phone plans in the US, and I have absolutely no trouble believing that people would subject themselves to a very bad messaging experience if that's the default that comes with their phone, as long as it's free. If I were to come up with a social/economic theory to further back that, my guess would probably be that phone users in the US have a "full service mentality" much more than in the rest of the world: Buying a phone on a (very expensive) monthly plan that subsidizes the device is much more common than elsewhere, and historically, SIM cards weren't even a thing, so devices were tied to a given network and (to some extent) even a subscriber much more than in the rest of the world. That would logically extend to people not expecting to have to install a third-party app for what they consider to be "basic phone functionality".