Because they lose to an ineffective tactic, and it frustrates them.

Which is why dying to swarms is so frustrating. Essentially you come face to face with your own ineptitude.

Camping wormholes is not as ineffective as people put it, it just has a random element factoring into it, which can be frustrating for both sides.

That said, pirating is supposed to be difficult. I remember the days when everyone pirated, and let me tell you, that didn't work. Obviously not. I'd much rather have a few good pirates than a hundred mediocre ones (the way it is now seems good to me, by the way). That makes encountering one actually a challenge, provides thrills whilst trading (I always fall asleep during trade runs with no pirates on) and generally enhances game play. Of course, all of this is just my opinion.