The level of decentralization in a blockchain network is the basis of its credibility. Decentralization ensures immutability of the ledger. For example in the Bitcoin network it is ensured by thousands of anonymous nodes independently functioning in the world.

How do we know they are independent? Well we don't. We can see their IPs when they post blocks, we could infer that their diversity in the world means decentralization, but IPs can be faked. Hence, here we rely on a hypothesis, that such a massive hash rate is not easy to concentrate in one hand, as it not easy to fake computational resources needed to produce blocks. Thus, the scale of the network itself gives us knowledge about decentralization.

I might be wrong here and without concrete figures it looks like pure speculation.

The problem with PoS is even deeper. To mint blocks you just need coins which you "stake". It is a lottery where you have more chances by staking more coins. How can you know that these coins are not controlled by one entity?

In PoS decentralization can be faked by spreading coins among billions of addresses, while one entity will still control majority of the coins - and hence will control the consensus.

Unless, there was a public pre-mining and all coins were allocated among large number of independent holders. And to say that they are independent the holders must be publicly known and have an unquestionable reputation. Surely, after that per-mining they all arbitrarily toss the coins and we will no know who owns what, but will have a bit of hope that the coins did not get into one evil hands.