I grew up in Loudoun county and lived in prince william county for a bit. It’s important to note that the wealth in Loudoun, prince william, and Fairfax counties are concentrated in pretty specific areas. Outside of Ashburn, South Riding, and the western estates of Loudoun it’s not terribly rich. The data centers bring in a ton of wealth though on top of the very large government contracting companies such as Lockheed, Raytheon, GD, etc. Additionally, DC and the surrounding cities just don’t have the density like other cities do, which spreads the wealth out a bit more (while still concentrating it in various counties around the area).