"Learned helplessness is a phenomenon in which after experiencing pain or discomfort in an inescapable situation, an animal or human will cease trying to avoid the suffering. They have learned that they are helpless - they believe they have no control over their situation, even if there is an opportunity to escape. This kind of conditioning was famously studied in Seligman's Learned Helplessness Experiment."
I think this is the reason. Scaling this up to human complexity, through prolonged suffering, misfortune, failure etc. a person learns that their actions will have a negative outcome or not change the outcome.