Why do you believe there is such an assumption? Elon Musk assumes people will try to achieve a post-scarcity society and he’s nonetheless gone from living in a YMCA to being the richest man on Earth.
He comes from money, and would have had no problem living anywhere he liked.
Just read his wikipedia page:
> In 1995, Musk, his brother Kimbal, and Greg Kouri founded web software company Zip2 with funds borrowed from Musk's father.
If the YMCA story is supposed to signify "rags", then it's a carefully constructed image and nothing else.