That’s a fair point and looking at most of the countries on the list with higher ratings, it looks like they are more socialist and have higher equality.
No doubt in my mind that if you are wealthy in the US you can have a very good life, but on a whole and for most people there it’s probably fitting that it’s lower down the list for quality of life.
Sorry if I don't take numbeo.com all that seriously. If you do, you should poll people to see if they'd rather live in Oman or the US and think again.