Because the games are fun and for some of us, the graphics remind us of the games from our youth. I also find that a lot of these so-called kiddy looking games actually have more of an original art style as opposed to something like all the popular FPS games and third person action games which seem to use all the same colors, the same looking environments and the same protagonists. Mario, Sonic, Crash Bandicoot, Ratchet & Clank, and Banjo all have a distinctive look. You can tell those games apart from one another. You can tell those characters from one another. If you were to remove the titles from the games, could you really distinguish most Call of Duty games and most Battlefield games?
I've noticed that with movies too. I'll often go see a Pixar movie by myself and notice that it actually looks different than other movies and not just because it's an animated movie. It doesn't use that disgusting teal and orange effect that most live action movies use these days. Has anyone else ever noticed that. Most action movies these days look like a Micheal Bay film, even the good ones.
Check out my art blog: http://jon-erich-art.blogspot.com