The only truly selfish reason not to cheat that I can think of is that it is not always possible to get the answer from somebody else in the real world. Your colleagues aren't going to do your work for you. You can't usually "just Google it". If you can't carry your weight now, why do you think that you'll suddenly be able to do so in the future? You're setting yourself up for failure by passing the buck to future you.

If you're looking for a rational reason to be moral in general, I would check out Kant's categorical imperative. It's pretty neat. It's part of why I don't ever cheat.