I don't think you understand cost of living as well as you think you do.
https://income-inequality.info/
Earning $30k in the US puts one in the top 5% of the world's population with respect to income (adjusted for PPP).
It tells me that rent for a 1 bedroom in Johannesburg is around $440 USD per month. In Denver, a mid level American city, you would be paying $1790 usd.
Also keep in mind that if you want health insurance in the US, you might end up spending $5k-$10k for just the insurance, getting care for anything beyond a yearly checkup is going to cost more.
We also have a very high income tax rate. Up to something like 45% depending on your level of income.
If I for instance saved 10% of my salary vs someone earning comparatively the same in the US, I would have far less buying power in the world in general.
Overseas holidays are something very few can afford. Spending ZAR overseas is rough.
Even within the states there are massive discrepancies. $15/hr in San Francisco probably means you are on the edge of homelessness. Meanwhile that same pay in the southeast might be enough for a modest life.