SMS was chosen because everyone has it already. So it's not a matter of being forced to have it, it's a matter of "all other alternatives are less widespread."

I think there are actually more people globally with access to mobile phones than people who have access to basic sanitation, i.e. flushing toilets.

I'm sure that many or most of those organizations have alternative, non-electronic means of interaction - such as phone, paper (writing checks), or in-person. That is the alternative to using a SIM card.