One of the top answers is: "Because of Digital Rights Management (DRM)..."
It's not scientific by any means, but I hope Brian O'Leary and others who are doing scientific work on piracy in the publishing industry might add this angle to their analysis.
That is: asking the pirates why they pirate.
And here is Cliff Harris' "Talking to Pirates" page, in which, on the subject of DRM, he writes:
People don't like DRM, we knew that, but the extent to which DRM is turning away people who have no other complaints is possibly misunderstood. If you wanted to change ONE thing to get more pirates to buy games, scrapping DRM is it. These gamers are the low hanging fruit of this whole debate.
Note that his findings regarding pricing are also interesting: he dropped his prices, and is still selling the same number of games, just making half as much money.
In any case, the publishing industry, about to start tilting at the windmills of pirates in earnest, would do well to study DRM's successes and failures elsewhere, and I hope that they make their decisions based on facts and data about what is best for the business, rather based on moral abstractions and vendor pressure.