"Both Bitcoin and Ethereum mining are very centralized, with the top four miners in Bitcoin and the top three miners in Ethereum controlling more than 50% of the hash rate.
The entire blockchain for both systems is determined by fewer than 20 mining entities."

In other words: decentralization in the crypto space is a myth, just as it is a myth that cryptos are currencies at all.

http://hackingdistributed.com/2018/01/15/decentralization-bitcoin-ethereum/