There are two kinds of wealth - private wealth and public wealth.
Private wealth is the things that you own, the money in your bank account, the car you drive, the house you live in, etc. For that kind of wealth, Americans are very well off, especially compared to the rest of the world.
Public wealth is the things that you share with others as a society, and how good those things are. It's things like the state of infrastructure, the ease of getting healthcare, the quality and safety of public spaces, how people feel when they're not in their home. For this sort of wealth America is a low way down the rankings.
Unfortunately for Americans, it's the public wealth that actually makes you happy unless you're in the 1% of the 1% who can afford to avoid public spaces entirely. If you own a luxury car but the roads are terrible that's just a reminder that your wealth can't solve all your problems. If you can afford to go to the fanciest restaurants but you're scared to wait for a cab outside afterwards, you're not going to be very happy with the meal. This is true for most aspects of life. Society needs to feel safe for people to truly feel happy.