you answered your own thought.
“poor” people spend their life trying to survive—they dont have the “luxury” or time for depression and mental illness—and if they are truly mentally ill, they generally wont survive in the herd for too long.
“rich” people spend their life trying to justify their existence leaving a lot more room for “thoughts” to get into their heads, spiraling into full blown mental illness.
I’ll sum it all up with a rough quote from William Holden’s last movie (i think), The Earthling:
“If you ain’t surviving, then you’re dying.”
Or something like that!
I have observed mental depression is more prevalent in entertainment stars. May be it is because they are showered with wealth and feel guilty about so much with so little effort, and do not understand why they are not more happy.