Email

1. Spammers invest in ways to get their email past your defenses.
They have a two-pronged strategy. First, they misspell words, use images, and do everything they can to get past the text filters that most people use. If they went to all that effort, but sent their email messages from only a few computers, it would be easy to knock those computers off the internet or blacklist them. Therefore, spammers employ a second technique—sending their spam from many different "zombie" computers. Because spammers control a shifting pool of hundreds, thousands, or even hundreds of thousands of zombie machines, their operations are very hard to shut down. Spammers may live in places where their money goes further than it would in New York City. It may make sense for them to invest months of effort for a few thousand dollars. This "cost advantage" enables attacks that many people would dismiss as not worth the effort.

2. A lots of countries where spamming is not illegal
Some countries have made sending spam illegal. The problem is that in lots of other places, spamming is not illegal. Even where it is illegal, there is competition for police resources. Spam is not as important to the police as a mugging or an assault. Catching one spammer takes much time and effort, and the payoff may seem small, especially if the spammer is in another country. Thus, the risks to a spammer are low, because the deterrent effect of the law is so small.