This is essentially the same as asking why can’t a country only use one person as its president. The idea of blockchain is removing the central authority by let

This is essentially the same as asking why can’t a country only use one person as its president. The idea of blockchain is removing the central authority by letting everyone has a chance to be the authority for a short period of time. If a person stays in authority for too long, the person may be corrupted and threat the entire chain. Therefore we limit her power by only allowing her to govern one block. And switch to a next person for the next block.