Because being poor is a complete absence of freedom. If you want to get out of it, you need to follow extremely rigorous lifestyle (like the one published by MacDonald) for decades, with each mistake costing you years of saving.
That is doable in the country side, but in urbanised area where you are bombarded 24/7 with adds, tempted by very easy credit, surrounded by toxic peers in a society that hates you ? If you think it is, then consider that you also start with a 13 IQ handicap().
Picture that as trying to be on a very strict diet while working drunk in your favourite sweet factory, with everybody around you gorging themselves, even those that shouldn't.