Because there is no _intent_ to slander.
https://en.wikipedia.org/wiki/Actual_malice
https://www.findlaw.com/injury/torts-and-personal-injuries/w...
That's facebook's situation. Even though they've got hundreds of billions and could easily afford to, they don't want to pay humans to verify these things and they know that innocent people are being hurt by their terrible lazy code, they just don't care.