PoW means you’re doing work to support XMR, so any XMR you get from mining is deserved. Yes, bigger entities have an advantage in doing more work, but so do off-grid mining with cheap/“free” energy in cold climates. It all balances out.

With PoS, the more you have, the higher your validation rewards, and the higher the rewards, the more you rewards, the more you have, which leads to even higher validation rewards. This rapidly leads to a few players dominating and the rest of the people fighting for crumbs at an accelerating rate. Plus since the big players are the only ones getting rewards, the little guy has to stake with the big players, increasing the profits for the big players (hooray for capitalism!). However, if there is any slashing, the penalties are spread to all the little guy stakers (hooray for socialism!). This leads to a situation that encourages big players to go for a 51% attack since if they succeed, they get wealthier and if they lose, their losses are absorbed by everyone else.

This can happen in PoW too (see Blackrock and Michael Sailor), but even there having a lot of the coin does not give you more power, only doing the PoW, so the growth is slower and the extra XMR doesn’t give you more power over XMR. Having a tail emission like Monero does should be enough to ensure that eventually no matter how much you have, it will be diluted by what’s yet to be mined.