Email is a reliable, negligible cost, vendor neutral communication system that's accessible to about two billion people worldwide. There has never been anything quite like that in history, and a mere handful of spammers (probably thousands, definitely not millions) are making it unusable. If those spammers had instead each gone on a local crime spree, any carnage and mayhem would have gone completely unnoticed by 1.99 billion of those people.

So they have a global effect, true. But with spam filters that effect is very small. One or two spam messages get through now and then, but you delete them and move on. If you get murdered you stay dead, people around you mourn, and your neighborhood is perceived as less safe.