It doesn't cost a whole lot to set up a pirate station. If you're using a computer to source your audio, you already have what you need for internet streaming. No one ever said a pirate couldn't have an internet feed.
Besides, people have to seek out content pretty specifically online. There are thousands of choices, so stream listeners usually know what kind of content they are looking for. Not the same with most radio listeners. They hit the "scan" button and sample what they encounter. If they like it, they'll stay and set a preset for your station. If not, they scan on until they find something listenable. Internet streaming does not have this type of listening. Therefore, radio allows the liberty message to reach the ears of the people who were not seeking it out in the first place. That's incredibly valuable.