You can automate it, but clearly that doesn't catch everything and automated systems can result in just as much bias as humans doing the job.
You can hire humans to do it and make them work fast, because the amount of posts per day is extremely high. You can use some automation to help them, but that will combine the problems of human moderation and automated moderation.
You can hire lots of humans to work at a normal pace and give them psychological counseling, and spend so much on their salaries that you'll go out of business, and even then they won't catch everything and will be frequently accused of bias.
It's a no-win situation for Facebook because even if they were able to objectively and fairly moderate every single post, they would be accused of bias by politically motivated people anyway.
This is not a technology problem, it's a social and political problem and Facebook will never be able to please everyone. Twitter is in the same boat.