This is my opinion.

Master Nodes are like the present day banking system, third party players, they deal with your money for you, not you dealing with your own money, so they are quite the oposit to decentralisation and crypto was suposed to be about getting rid of the middle man, that is the simple answer, they take controle away from you, making you dependent on them and they get the lions share of the rewards from staking or transaction fees, so they hold all the cards and you pay them for the privilage.