You'll always have centralisation in any system, you're just trying to minimise it as much as possible. Mining operations require you to continuously supply to the system at cost, which is completely different to staking that requires extremely minimal to no cost for operation.

Both PoW and PoS have centralisation, but PoS vastly increases this centralisation, does it at a far faster rate, and "locks it in" (there is no feasible way to overcome the most powerful node in PoS, this is not the case thankfully in PoW)