It's both a myth and a fact.
It's a fact that the US is one of the wealthiest countries in the world, with one of the highest per capita levels of income, though not everyone in the country benefits from that average ; many are well below it. So it's a myth since not everyone in the US enjoys a privileged lifestyle ; many people don't, even by average global standards.
Much the same can be said of many other Western countries.
Of course, material wealth is not the same as quality of life, and there may be many countries with a higher score on this basis than the US [or the UK].