People have this insane notion that one line of defense is enough or they have the equally insane notion that no wants to take something they have.
People have a really hard time evaluating simple risk/reward models. You have to make the reward of attacking you higher than any possible reward to have a reasonable chance at security. If anyone anywhere on the Internet can profit by your loss, eventually someone will try.
EDIT - To make life even more difficult sometimes the attack provides gains indirectly. Imagine one group of politicians attacking a service supported by another political faction, just that service going down profits the first group if it changes who voters vote for.
People have a really hard time evaluating simple risk/reward models. You have to make the reward of attacking you higher than any possible reward to have a reasonable chance at security. If anyone anywhere on the Internet can profit by your loss, eventually someone will try.
EDIT - To make life even more difficult sometimes the attack provides gains indirectly. Imagine one group of politicians attacking a service supported by another political faction, just that service going down profits the first group if it changes who voters vote for.