What do you mean "don't work?" Do you mean "Don't stop 100% of spam?" If so, walls "don't work" because things can go through them, body armor "doesn't work" because missiles exist, and boats "don't work because they can sink. Do you mean makes more difficult to spam? Because yes, they demonstrably do. Side channels are expensive, andusing gossip with filters requires massive coordination. So what do you mean "filters don't work?"