PoW and PoS both allow a single party to unilaterally control the network when they're working exactly as intended — just under conditions that are impractical in major networks. That impracticality can be measured in economic terms — as a dollars-and-cents cost to acquire the assets necessary to take control.
It's harder to buy out 51% of validation stake of a network of size N, than it is to buy the GPUs and the electricity to power them to do 51% of the mining for a network of size N. GPUs are technically unbounded — you can just build your own fabs, if you really care — and so the marginal cost of acquiring another GPU+electricity is constant. But stake is a pie; the marginal cost of acquiring more of it increases as less of it remains available on the market. (And many people will just never give theirs up, for ideological reasons.) Thus, it's harder to Sybil attack PoS than PoW.