That would make sense if every message were placed on hold before publishing while it got reviewed. But this is a real-time system. And even then, there are often multiple sides to events.
Is it a hard problem? Sure. Can they get everything? No. But that doesn't mean that they can't quickly act to minimize the total impact. As with many things, 80% of the value can be gained with 20% of the work.
The hard question is whether 80% of the value is available. If you restrict 80% of the worst offenders, but the selection of the 80% is biased in some obvious way, that may be worse than doing nothing at all.
I'm not saying it's easy, but effort should be made as it is important. If you see the real-time voice recognition on the new Pixel phone, the technology for this type of action might not be that far away.
