Middle class not rich. Getting out of being poor is hard on its own. Getting from middle class is "easy" in that random children from middle class raise to rich all the time. However poor have other disadvantages that they need to deal with, and that is hard to do in one generation.
And most billionaries are first-generation: https://www.cnbc.com/2014/07/16/how-to-stay-rich-for-three-g...