Because it's false. There are many, many, many examples of entirely uncontrollable life circumstances rendering one's life shitty. Some people recover, and some don't. Those who don't shouldn't be automatically blamed for it, as a substantial portion of the variance in personality is heritable and providing a case study of someone recovering from an equally bad or worse situation definitely doesn't prove that anyone could do it. Many individuals with better circumstances refuse to accept this, as doing so would potentially lead to a grimmer worldview.