In practice a PoS network tends to have a participation rate of between 10% and 30%, and the 2-3 largest exchanges collectively tend to own between 10% and 50% of the total supply.

And then also in practice most of the staking is performed by a small number of outsourced staking firms, which increases the power concentration even more.

And it also makes hardforks more traumatic because your economy and your consensus builders are fundamentally intertwined. You can't fork an exchange off of the network without also disrupting every single user that parks funds on that exchange. The same concern doesn't exist with PoW mining.