Why would you say that? Do you think that rich people are happy and poor people are depressed? Anecdotally, the poorest people I know are often the happiest and the richest people are often the most miserable. I was a whole lot happier when I was younger and poorer. One thing has nothing to do with the other.
There are numerous studies showing this, for almost all disease categories. All psychological illness is negatively corelated with socioeconomic status.
https://en.wikipedia.org/wiki/Socioeconomic_status
I'd say it's hard to find good sources of nutrition and the time to exercise when you're part of the lower class.
edit: negatively correlated...
Also depression. It took me one minute to find three sources, could you please make some effort here? I have other things to do.
And you're right, it's not a causative factor necessarily, but I think there's pretty strong evidence at this point, it's hard to find a poor person who doesn't have some condition.
[1] https://www.sciencedirect.com/science/article/abs/pii/S00223...
[2] https://psycnet.apa.org/doiLanding?doi=10.1037/0021-843X.111...
[3] https://ajph.aphapublications.org/doi/full/10.2105/AJPH.93.1...