Where are all the people who grew up too poor to get dental care ending up? Not in high-end tech jobs, and tech is less highly selected on this than finance, law, etc.
I've interacted with quite a few interns and new grads at big tech companies and, among U.S. born folks, I think the rate of people who were on the U.S. Olympic team is pretty similar to the rate of people who haven't had dental care (IME, the Olympic team rate has been higher, but sampling from tails like this is noisy)
For a less extreme example, a large group of tech folks I was hanging out with compared notes on US high schools. My high school was ok (one random site ranked it 35%-ile among schools in WI, another one rated it well above median), which made it bottom of the barrel among tech folks. Most people went to high schools where basically everyone graduated, very few people were in the "reduced price lunch" income bracket, etc., which means they were in high-end high schools.