I think that is one of the great weaknesses of PoS and one of the big points for PoW.

Not really, the difference between POS and POW is that in the later the block producers are elected by those that can contribute more hashing power as opposed to having more stake. The fact that only a handful of mining pools are effectively able to produce blocks should be enough evidence that POW also leads to a monopoly (technically an oligopoly).

In POW each individual miner has to decide to which mining pool they should direct their hashing power. The rational thing to do is to choose the one that gives them a better chance of getting a higher portion of the block rewards. It turns out that the best choice is to join a pool that already has a high percentage of the block production.