The cause is disparity.
Yes, adversity does not cause prosperity. When it happens by chance, they write books and make movies about it. No good can come of making people struggle.
Serendipper, I love how we continually concur with one another in most major ways, but let me qualify your closing sentence.
I think the difference is between a surmountable struggle an an insurmountable struggle. Struggle is of course what causes success, achievement, progression. It’s surmountable and in such cases where people succeed, this is provably so.
But the problem comes when people abstract the type of struggle from its context (usually through ignorance and/or laziness). It is generally assumed that if someone can surmount a struggle that is worthy of a book and/or movie, such a struggle must be generally surmountable. Of course under some (often most) contexts this is not the case.
Perhaps you have become aware by now that I am absolutely behind the kind of work being done by such people as Robert Sapolsky and Sam Harris on such things as the notion of free will. This is nothing more than the acceptance of mind-body causation - at least in the direction of the body being the thing that affects the mind (perhaps being considered as the same thing: Monism, or even not considered as working in the other direction: Epiphenomenalism). Is the mind fully the arbiter of the body’s actions and is there no or at least some influence of the body on the mind? Experimentally it’s becoming increasingly clear that the mind is fully dependent on the body - your free will is nothing more than that which was determined by your body’s interaction with your environment (neither of which you initially chose to be born to/into).
Give people surmountable challenges and they grow, give them insurmountable ones and they remain in poverty and too often turn to violence and other anti-social behaviours.
Me too! 
Yes you can see the situation clearly! That is exactly right: providing what others could provide for themselves is an impediment to progress, but it’s too easy to get carried away and think that the cause of progress are the impediments such that one begins to equate adversity with prosperity.
For instance parents often do too much for their kids in the way of cooking, laundry, etc so that kids never learn independence; they coddle too much and raise dependents without survival skills. That is true, but other parents take the philosophy to extremes and believe that specifically making it harder on kids produces kids that are more self-sufficient, but if that were true, then the hoods would be churning out geniuses and poverty would be self-eliminating.
Obviously there is a middle way between the two extremes: we need to be challenged, but can’t be overwhelmed. This is true in bodybuilding since sleep is more important than gym-time: sure we need the stimulation to trigger muscle growth, but we also need to coddle ourselves while the muscle responds with growth. It’s true with the immune system as well: we need to be challenged as babies in order to develop a strong immune system, but we can’t be overwhelmed to the point of death. Likewise it’s true with society: we have to guarantee everyone basic necessities lest they be overwhelmed by insurmountable challenges involved with mere survival instead of having the opportunity to contribute to society in a more meaningful way.
Organisms reproduce more aggressively in response to poverty (environmental stress) which increases the odds the species will survive by betting on a genetic variation that will overcome the environmental stressor. That strategy produces strong animals, but it also produces lots of dead ones and humans would find it abhorrent if people were allowed to starve in order to genetically evolve into people who can better-handle adversity just so the rich can keep their wealth because they are the most productive of society which means what; that people have to starve to make it so? That doesn’t seem so noble. People don’t need to be so reliant on genetic strategies since we have technology. We just need to open our eyes to the fact that the machines are working for the few instead of society and make the inevitable moral judgement that each of those machines should be paying taxes like the workers were before the machine replaced them, then that money used to relieve people of drudgery and provide a socioeconomic floor that no one is allowed to fall through.
Yes it’s hard to escape the fact that we are continuous with our environment. I could argue that my blood is my environment or that my room is my body because I have more control over the temperature and humidity of my air than I do my blood pressure and most of the things happening in my body may as well be something in outer space considering what little control I have over it. I have about as much control over my thyroid gland as I do the sun.
Warren Buffett attributes his success to luck because he was born in the right environment for his genetic propensity to thrive and if he had been born in some other time or place, he says he would have been some animal’s lunch.
People flatter themselves saying “hard work is the key”, but how did they get that ethos? Luck? Whether one is smarter or more diligent or whatever reason they cite as responsible for success, possession of that attribute will always come down to luck. I didn’t make preparations to be born how I am, but I just woke up this way. That’s what my name means: I stumble into things by chance and it’s no credit to me.