Almost all good things (books, software, music, movies) get pirated. If nobody pirates yours (even as time passes) this is a very negative signal - chances are nobody wants it and nobody finds it worth sharing (or almost nobody just knows it exists). And the pirates mostly are people who would never buy anyway so this is not really a material loss. Some of them even pay once they find the book worth it and once their budget permits. I tend to buy paperback copies of books I have found online and actually read, and I have hard time resisting temptation to share books I've bought digitally as great books feel like they would make the world a better place if more people would read them :-)