The first reason is just because they can do it, no matter how absurd this sounds to you but this is a fact. Hackers and uploaders get a sense of importance by doing so, they crave for the recognition and following of their fans. The cult status of various release groups and importance they get in pirate world is the foremost reason for people to get involved in piracy. It is just fun and excitement for them but for the developer the loss and effects happen in real life.
The large amount of advertising revenue generated by the websites where links to pirated files are posted is second biggest reason. Take example of Rapidshare, it is one of the most popular webhost used by the piraters to upload their files and is ranked 17th worldwide in web traffic. Although Rapidshare has no searchable index of hosted files nor it advertise to downloaders but imagine the number of surfers looking for pirated content and then imagine the number of pageviews websites in this segment are generating. Although webhosts like rapidshare do best of their effort to remove pirated files but uploaders disguise the files under false names and password protect them, making it difficult for webhosts to detectt he contents.
Hackers or cyber criminals release modified copies of applications to deliver malicious code to unsuspecting users. When a surfer downloads a pirated copy infected with virus or a trojan horse his PC gets compromised and is at the mercy of hacker. Modified applications normally do not work as advertised on the publisher's website or behave abnormally when used. Although user knows that he is using a pirated copy but he still expects the application to perform perfectly. The end result is that you lost a customer due to piracy and also got bad publicity without any fault at your end.
In some cases a fourth reason of foul play is a competitor or an unhappy former or current employee. The existince of beta version or pre-release copies easily found on the internet points to the fact that an insider was involved in leaking a copy to pirate world.

