I’ll go out on a limb and say that amount of people willing or even able to do so is totally insignificant. Retiring to another country amounts to becoming stranger in a strange land where they may speak some English, with vastly different healthcare system and healthcare practises, having to learn and unlearn at least some customs or manners, leaving your family, friends, networks and safety nets behind. Starting over in another country can be life changing, but I believe that for most people it is simply not something they are willing to go through, especially during their golden years.

Oh and there is an added bonus: having to deal with stateside banks wanting to close your brokerage accounts as soon as you let them know you're about to move abroad with unknown intent to return. And finding it near impossible to open brokerage accounts in foreign banks when they learn that you're an American citizen. Because FATCA. (to be fair, I believe this depends a bit where you're relocating to, but at least with Europe, no dice)