I’d argue the SMS/MMS reliance in the US is entirely because there have been no caps on it for years now. Nearly all plans you can get here have unlimited SMS/MMS included, even cheap prepaid ones.
Having a fixed allotment of texts or minutes hasn’t been a thing for over a decade at this point, and the only thing that’s expensive now is data.
Right, nearly all US carrier and MVNO plans have offered unlimited SMS and MMS for years now. It’s free, it’s built-in, and it’s easy. For most, it doesn’t matter if chat apps are better, so it’s been very difficult to convince people you chat with to switch to a different app.