The posts you're talking about will either be buying their votes or using their larger accounts to farm rewards. They'll openly admit to doing it (due to the history of Hive) and won't stop. It needs somebody with even more power than them to downvote until they stop. And that hasn't happened yet.

isn't Steemit decentralized?

The fact that it's decentralised makes it possible. If it was centralised, the owners could delete the accounts.