The main reason is that games on GOG.com are cheap. You won't see many $50 games given away, but $6 giveaways don't harm one's wallet significantly. Notice that you don't usually see many $10+ giveaway on here either. ;)
Materialistic reasons aside, when people like a certain game really much, they want to share it with older people. Who among us hasn't at least tried to show our favourite game to unenthusiastic friends or spouses? In the same way, we tend to spread the gospel of a great game to everyone who might have missed it. Furthermore, people who have won something in a giveaway feel obliged to do the same for others.
Finally, it's a win-win-win situations. Winners get games, people who give games away get this warm, fuzzy feeling, and GOG.com gets more revenue!