Except they're not. The vast amount of spam comes from a few IP blocks who are so-called "liberal" in their spam policy. That is to say, they're spam paradises and earn their money from those shady customers while the vast majority of the world blocks these. Not via police force, mind you, but via network/sys admin force.

The rest of the spam is done using compromised computers but once that's done the ISP of that provider gets quickly noticed and either the ISP blocks that customer or they themselves get blocked wholly due to their inaction. So they choose eggs for their money (as the Dutch saying goes).