Because a 40 minute demo doesn't represent a 15 hour game. It's skewed towards promoting just the good parts, and some companies are plain better at making demos than others.
I want to give my money to companies that make good GAMES, not just good demos.
+1 and in a lot of demos you can not change the settings that would have made it a great game.
Ps. I think charging R600 for a game is madness.