Blockchain consensus is based always on a scarce resource, like at proof of work this is the computing power, at proof of stake this resource is some kind of a token that has a limited supply and kind of monetary value, at proof of authority or byzantine fault tolerance this resource is kind of a right given by a decentralized authority for voting. The reason for the need of the scarce resource is that the sybil attack, like replicating a node a couple of million times on a cloud environment, can not work because participating on the consensus depends on the scarce resource and not on the number of replicated nodes.
