Surely many who have them received them from elsewhere before immigration to America, and likewise the proportion of immigrants who have them I would expect to be oversized. Americans tend to be more greedy than anything else and don't put in the effort required for such small (financial) rewards.
Also, those with PhDs tend to congregate into certain areas that support those jobs, i.e. cities but not even a goodly number of those so much; plus smaller college towns too ofc. As such, many in the general populace might rarely if ever run into one for the largest majority of their lives, unless traveling specifically to those areas for some reason?
And ofc rural areas are far larger, geographically speaking, than places where a person with a PhD would (likely) go. So you could randomly pick a spot on a map 100 times and never manage to find someone with a PhD anywhere within tens of miles, I would expect - although that line of thinking reveals my own biases: do most educated farmers stop at like an MS and just follow up with their own (possibly even extensive) self studies, or go all the way to PhDs while working their actual farms? (I doubt it bc it does not sound practical, and that is a hallmark of farmers afaik, but I could be wrong...) Anyway, I expect the unequal distribution is a contributing / exasperating factor to the general rarity.
Little by little, progress is being made - in the meantime the software isn't quite "finished" yet, but somehow still the UX ends up being significantly better than Reddit:-).