Richard Easterlin found a correlation in 1974, and subsequent studies have reinforced that. See the Introduction in https://pmc.ncbi.nlm.nih.gov/articles/PMC9802463/.

Stereotypes of extrema in wealth are attached to images of extrema in happiness. The poor sad person vs the rich happy one. Cliché are often great tools to make quick judgment, but of course quick judgements often fail miserably when it comes to scale the idea.

Because it is mostly true? I've seen wealth and happiness in society a lot more than poverty and happiness.

Because research on this topic supports it. Happiness and wealth are correlated.

It's how Americans think life works (I've fallen victim to it as well).