One motive I haven't seen mentioned yet:


Kingdoms and merchants with any sense will send protection along with valuable ships. A single pirate ship, no matter how skillfully captained, is not going to be able to challenge a convoy of well armed ships, no matter how tempting the prize they guard. But if that single pirate ship can contact any nearby pirate ships, and be guaranteed that any betrayal would earn the ire of the rest of the confederation? Maybe even have a set policy on organizing for combat and distributing loot, so it doesn't take much time to coordinate an attack? Suddenly the naval lanes are yours for the taking.