PoS is about having a billion to buy the factory, lay back, and make all the decisions without doing much else. The ones who already have the money will hoard power (in the non-electricity sense) around them. They will use this to make more money, buy up more, and centralize the operation even more. As most coin supplies are limited, a majority is a majority; and if you control it, nobody else will get into the role of having more. (Stock companies work like this as well: selling minority stakes but making sure others never reach voting power). It's a means to make sure those who have control will keep it.

PoW is about actually gathering the wood and building the fence. Of course, you can also buy more power, but so can someone else; power isn't coin supply. Also, a consensus mechanism makes sure that even if you have a good chunk of the supply, you're not actually controlling it; and because power is NOT coin supply, you can just cut off the majority producer if they do something funny. This has long been debated about bitcoin: what f a state actor drops a load of energy into mining to control the network in a 51% attack? Well, the 49% could just split from the network, declare whatever that 51% actor does "not real bitcoin", and then watch how that state fork does.