Has to be? Don't know about that. I have 41 years of experience in the human condition, and 34 of that as a gamer. Having pirated in the past myself (and anyone who has been a PC gamer as long as I have probably has too), I can say that your reasons for pirating games varies, often from game to game. What I can tell you is that 'piracy as a demo' is something I did often. Did I always buy every game I liked? Nah, but I did it often enough. If I still used it as such now, I definitely would pay for everything I liked and delete everything I didn't because I want to reward the developers doing it right.
The thing is, I don't pirate games anymore **
because of things like Steam sales and Humble Bundles (which is how I acquired World of Goo, incidentally). When there is virtually no financial risk, and when buying is actually more convenient than pirating, a "once-in-a-while pirate" like me can be changed into an "always-a-customer". Who's to say that it doesn't happen for millions of other pirates the same way?**
I still refuse to be treated like a thief by Ubisoft, so after I buy games with their always-on crap, I play the superior warez version. This is the only case where piracy still applies for me.
“The greatness of a nation and its moral progress can be judged by the way its animals are treated.”
- Mahatma Gandhi