Mistakes happen at scale regardless of whether it's automated or human-controlled. Having actual humans do the work is hardly a magic bullet, because people make mistakes frequently as well. Worse yet, humans are more prone to being compromised or acting maliciously, such as in recent attacks on prominent companies.
It's not so much as a policy as a necessity. Facebook's number of users is counted in the billions. They have about 50,000 users for each Facebook employee.
Most importantly: People aren't paying Facebook. It's just not realistic to expect a free platform serving billions of users to have 100% perfect execution.