zohaibhisam: the "Theme" plays strongly into game rating. Since the theme is not known prior to the start, it would be difficult to work on it before time. Submissions end after 49 hours (48 hours + 1 hour submission window because the site is always hammered at the end) so you wouldn't be able to submit after that point.
On top of all that, it's the honor system. If you have a game that does well and gets noticed, people will notice and call you out if you break the rules. It's not like there's a monetary prize.