They finally hit on the real story halfway down: "“Generally, it is clear that things that make life less boring are a priority for the poor,” they write in their new book." Yes, the poor spend their money unwisely when they have extra. That's the real reason. If a rich person finds themselves down on their luck, they knuckle down and work really hard to get into a situation where they can afford to buy luxuries. It…
Most very poor people are not young, athletic, with good physical and mental health, no dependents, and the background security of being able to leave their circumstances immediately any time they want. The book is "Scratch Beginnings" by Adam Shepard, btw.
Chicken and egg problem?