> Why would a scammer focus on targeting low-disposable-income countries, if they, on average, can extract as much money from one US person as they would have to from 10-15 people in Phillipines.

I don't think anyone disputes this, but apart from Canada (which has a largely similar telco structure) no other developed country has this plague of undesirable robocalls. You could probably net a similar payout in the UK or Germany but as far as I'm aware there isn't a robocall problem in Europe.