Honestly, it's because with the Sims you can do things you wouldn't, couldn't, or shouldn't do in real life, but you can in the game because it's just a game. Also because I have a vivid imagination so for me it becomes a story playing out (several stories really). And of course there's the creative aspect involved in making over sims, designing and decorating houses, so, yeah.....lots of entertainment!
Oh, and I love the genetics feature....so much fun seeing how different (or similar) each offspring looks!