Development agencies actually hire lots of local people. for USAID/State Department, they're called FSNs, and its pretty easy to find stories about them and the work that they do. According to one Document I found around 55k/75k state department employees were Foreign Nationals (aka local hires). [1] Foreign nationals don't run these agencies because these agencies are responsible to their respective governments and have missions and goals that extend beyond simple development. This post seems incredibly uninformed given that the extent of their research seems to be looking at the citation of the photographs/videos.
Even if we set aside any potential spiritual, or child-rearing value. Marriage is a very important institution in helping people achieve economic mobility, it increases your household income significantly, as well as seriously reducing your cost of living. Anecdotally, my cost of living was drastically reduced when my girlfriend and I moved in together (thereby splitting a 1BR), however given that we aren't married we individually have to save a good amount of money in cash accounts in case of a breakup. If we were married, we could reduce our cash savings by 50% and instead invest that money into the open market. Moreover, we would be far more likely to take care of the other financially if one went to school if we were married. Marriage, and the commitment it entails changes the attitudes of individuals towards each other by ensuring that we can plan for a combined future. all that exists outside of societally constructed benefits like joint taxation and healthcare.
digging into this, it doesn't seem to adjust for CoL but competitiveness of the market. Seattle and NYC have the same salary bracket, but NYC is by far more expensive. Nerd Wallet claims the city is between 37% to -6% more expensive depending on where you live (Manhattan vs Queens)
[1] https://diplopundit.net/2017/05/08/statedeptusaid-staffing-c...