I live in Arlington, so not Fairfax, but in a neighborhood that borders Fairfax (and not really that far away from TJ either). The median household income for our census tract was about a 3rd of the county's average, ~$40k vs ~$120k when I looked into it a few years back. Which is also indicative of the Fairfax areas around me. It's not like America has much in the way of generic social support to make it easier for folks like that to spend a couple thousand on some extra tutoring, plus the cost here is higher because there are so many affluent folks to drive up the price of a tutor. This also isn't a small number of people, it's a really dense area down here relative to other parts of Northern Virginia (including parts with substantially better transit access and more walkable communities).
Northern Virginia has some pretty unique pockets, my zip code is one of the most diverse in the entire country[0], and a bit south of me in Fairfax is the only other one Virginia has in the top 100, but also quite a bit of poverty in those same unique areas. Idk, I just wouldn't paint with such a broad brush, when I know very few folks on my block with kids could just spend a few thousand on extra tutoring.
[0]https://medium.com/@waldoch/measuring-neighborhood-diversity...