Is it not?
On the Bitcoin network, 4 block producers produce over 50% of all blocks among themselves. 8 together create 75% of blocks, and 12 produce 90%.
On Ethereum it's even worse, where only 2 pools produce over 50% of blocks.
The reply is often that there are more miners than there are pools, but individual miners do not produce blocks. They only allocate their hashpower to those who do, which is effectively a form of voting. Thus non-block-producing miners are the closest equivalent to voting stakeholders, not witnesses.
Japan and Russia are also going into the mining buisness. We will be seeing more decentralization quite soon. But you're right, China has mining monopoly now.