This is the reason Amazon's job is way harder than it looks at scale. Any system used to try and prove that an account is legitimate or not has difficulty telling when an account has been hacked/bought. A smart attacker will bide their time and will do very few falsified reviews. Everyone is trying to game the system, and beat the other "players", and are willing to resort to nefarious means to quash their competitors because money is on the line.
