Tons of studies have been done for the last few decades and they show a pretty solid consensus (not completely unanimous but it's pretty strong) that wealth and

Tons of studies have been done for the last few decades and they show a pretty solid consensus (not completely unanimous but it's pretty strong) that wealth and success in this country are very largely hereditary, that socioeconomic mobility is pretty flat (and very flat relative to other developed economies except maybe the UK) and that the American dream (rags to riches for anyone who works hard, i.e. social mobility based on merit and effort) is a myth. So no, if you're poor in the US it's not just because you've got a mental illness, that's a very wrong view of the world. Poverty is a big issue in the US and it is distinct from many (not all) developed economies for having a lot of working poverty, i.e. people who are not ill, do not suffer substance abuse, who aren't homeless and who have a job (often multiple) and yet are poor. That is very much an American phenomenon, no other developed country has so many normal people who have jobs and yet are poor and who struggle to leave their children a better life than they enjoyed.