Okay, basic economic principle here: the reason why a pirate wouldn't have bought the game is because it's not worth $X dollars to the pirate that they are charging for in the stores. That's just a statement of fact.
Now the question is why isn't it worth it? Is it because he can play quite a few other pirated games at a much lower cost (download time) ? If he/she didn't have other pirated games to play, how many games would he/she have bought instead?
Fundamentally, piracy is reducing the value of all PC games, because they can be obtained for a much reduced price. And how do you measure this loss to developers? It's not just specific lost sales, but an overall devaluation of their product.
Sure, they might not buy that particular game that they pirated, but if there weren't pirated software to entertain them, they would have bought something.