Several reasons.

1. The US is largely responsible for defining what 1st, 2nd, and 3rd World countries are.
2. It has the largest economy in the world.
   (I think? That may have gone to China by now. Not sure. But it was true recently.)
3. Even with everything you said being true. It’s still the wealthiest country in the world, by a large margin. Epically when you compare incomes, lifestyles, and infrastructure to actual 3rd world countries. It’s not even close.