Why is Williamson county, Tennessee so wealthy?

[lfromnj]

1) Why is it surprising that it is so wealthy?  I have always thought of Nashville as a fairly affluent city...

2) Why is it REMOTELY surprising that it is so Republican??  Lol.  It is a wealthy and white county in a moderate-but-not-huge metro in the American South ... those areas are nearly all Republican, and the ones that aren't are centered around very diverse and much larger metros like Atlanta.

It's an oddity in the sense that among the top 20 wealthiest counties, only 2 are GOP-leaning and among them is Williamson.

https://en.wikipedia.org/wiki/List_of_highest-income_counties_in_the_United_States

I'm not sure what data you were using, but I used this Wikipedia page that cited an American Community Survey from 2016.  This is just semantics, but according to this list, 8 voted for Trump in 2016:

Forsyth, GA: +47.60% GOP
Williamson, TN: +35.50% GOP
Calvert, MD: +18.40% GOP
Douglas, CO: +18.10% GOP
Delaware, OH: +16.10% GOP
Hunterdon, NJ: +13.80% GOP
Stafford, VA: +9.10% GOP
Morris, NJ: +4.40% GOP

Just for good measure, Nassau, NY was decently close (+5.40% DEM).  So, I would say the list is pretty split...?  Echoing back to my answer, a lot of the Democratic counties on that list are in large and diverse metro areas (every single one is either in the NYC area, DC/Baltimore area, Bay Area or Greater Los Angeles).  Of the ones not in those very large, cosmopolitan and diverse metro areas, every county on the list voted Republican.  So, it shouldn't be inherently surprising to you that a wealthy county would support the Republican Party, and it should be less surprising to you that a wealthy county outside of the biggest metros in the country would vote Republican.  I mean, exit polls show Republicans winning the wealthy vote in both 2016 AND 2018, and considering they lose wealthy counties in the biggest metros routinely ... those votes have to come from somewhere, right?  Throw on top that it's extremely White, in the South and (as Del said) mostly made up of married couples (a group consistently more Republican than the voting population at large), and it would be surprising if Williamson County WEREN'T very Republican.

As to your question for why it is so wealthy, I am not qualified to answer, but it looks like there are good ones up-thread.

TBH im not 100% sure about the wealthy vote being GOP for Trump in 2016 although I do believe it voted GOP in 2018, there was basically a 20-40 point swing in the wealthy vote across the nation, Highland park Dallas +65 Romney to +34 Trump,Mission Hills Kansas +40 romney to +0 Clinton,you can say sure these are all big metroes but even areas like River Hills WI was like +13 Romney and +9 Clinton, etc, or Bloomington Michigan etc even voted for Mccain but also Clinton. However most of these areas swung back to the GOP in 2018 for federal races besides Highland Park in the 2018 TX senate race.

[lfromnj]

Williamson is so wealthy because it's incredibly monolithic - it's 94% White/Asian and 70% of the households are married couples (over 40% with children at home).  Williamson is probably the most populous county in the country that is still entirely a demographically-stereotypical 20th century Leave It to Beaver suburb.  For comparison, Cobb County is only 66% White/Asian and 50% married couple households.

I will also note that while Williamson County has the highest MHI in Tennessee, it is not the most "elite" Nashville suburb.  High-power doctors/attorneys/execs with Ivy League/private college education and Nashville "old money" are still concentrated closer to the city center (Belle Meade, Green Hils, etc.)  This is a common dynamic in most American metroes, but the slightly more suburban, extremely monolithic areas (like Williamson) will end up looking wealthier based on the high-percentage of dual-earning, single-family homes relative to more urban areas.  

Further flung suburbs have more of your high income, dual earner middle management and reasonably successful business types living in SFHs.  The floor is higher.

Most of the elite/old money, professionals, law partners and so on is in inner suburban counties, but they're more mixed.  Hence Montgomery and Fairfax are more "elite" than Loudoun County which has the higher median HH income.  Westchester is more "elite" than Somerset County NJ and so on.

And the *really* elite live in the cities themselves, in places like Lenox Hill, Beacon Hill, Kalorama, Pacific Heights, etc.

Or at the very least basically within the city but a seperate town such as Highland Park Dallas or Bellaire Houston.