It can be. There is a button to flag posts. Moderation can prioritize by the flagging and the amount of flags a post gets. Considering how many analysis is done onto the facebook users, it should be relatively easy to come up with a scoring system. Could be as simply as how often a post by a certain user is flagged (and potentially removed by moderators). Also just randomly check posts can be effective, if consequential action is taken.
Yes, this is some effort and will cost money. But we are talking about hugely large and profitable companies here.
While it might not be possible to "solve" the issue entirely, it could get a lot better if done thoroughly.
They already do everything you suggest. Clearly not as well as people would like, but this is like Moderation 101. They get better all the time at automatically analyzing things to have humans intervene. It still sucks, they will never be able to keep up. People are monsters and don't deserve nice things.
Yes, this is some effort and will cost money. But we are talking about hugely large and profitable companies here.
While it might not be possible to "solve" the issue entirely, it could get a lot better if done thoroughly.