Apart from they most likely don't, right? This reason always comes up but I would argue that it's a fallacy.
Nowhere else in life is it seen as ok to steal something to see if it's to your taste. Nowhere else in life do thieves go back and pay for the thing they just stole because they find out they like what the stole.
You're not stealing from the rich to provide for the poor. You're just stealing from people who work unreasonable hours and endure high levels of stress to entertain you.
Like it or not it is known that game demos generally do not help sell more games. I know that the pirate games I played when I was younger, even the ones I liked, I never paid for. Even though I went into it with the same reasoning as you.
Here's some videos that I agree with to show you where I'm coming from with my opinion. Hopefully you can consider changing your approach to doing more research before buying games, instead of using piracy as your solution.