In PoW, the "rich" have to keep selling their coins to buy more dedicated mining equipment in order to stay competitive. And because PoW hasn't reached "optimum efficiency," there's still profits to be had by building better miners and cheaper power sources. Innovations here have positive downstream effects, since innovations in cheap renewable power and efficient chip fabs and designs have many applications beyond mining.

PoS, on the other hand, requires minimal extra work from the already-rich. They just sit on their butts getting richer.