Because, fundamentally, most people aren't assholes.
I vacillate between both sides of the piracy issue. I used to pirate, when I was 14 and had no money. About the time I turned 16 or so I realized it wasn't a good idea and pretty much went without if I couldn't buy it; I saw no reason why other people couldn't do likewise, and got pretty annoyed at the casual piracy at the time. Having graduated from college and moved into a nice job, I buy stuff pretty much on the basis of "it looks interesting and it's only two bucks," and I can't bring myself to care about the pirates at all anymore. They're not part of my worldview.
If you can make the process of buying something simple enough and pain-free enough, I think that most people who can afford it will buy it. Others will pirate it because they can't afford it, and I think that there's a compelling argument to be made that many of those will eventually become regular consumers when they've got the money to do so. There surely is a group of dyed-in-the-wool douchebags who will never buy anything they can pirate because they are, at their cores, Bad People, but them's the breaks--I've come to the gradual understanding that these people are shitty human beings who will screw others so long as they don't have to look them in the eye to do so, and all you can do is write them off.
But I think that, for the most part, if you treat your potential customers right, they'll do right by you. (The same is not true, I think, of "donate what you want"--I strongly feel that method of revenue generation encourages people to pay as little as they can rationalize, and what even nominally 'good' people can rationalize is really really small. But that's another topic.)