Cost is a big factor.

Those dual-sim and tripple sim phones are widely available in South Asia, and so many varieties are available that all you have to do is get the phone, buy whichever carrier's SIM you like, and off you go. Just keep putting credit/balance in it in local currency.

No such things as contracts, or billings, etc.....never see a bill.

Here in north america, each carrier wants to lock people in, with very expensive so-called plans and extras, etc.........they would lose too much money/jobs if this happens. Simply isn't feasible. Alot of people here don't even know the concept of dual-sim phones, generally.

General public would be switching sim's really fast if competition was so fierce, like it is, in those heavily-populated yet poor populations. They are fine with their networks and don't ask for 4G, 5G, 6G and so on like people here in the developed world.