Yes, to propagate the species. But why is this a good thing? In a purely material world, everything comes to the same end. It doesn’t ultimately matter whether your species lives for 50 years or 50 000 years.
It's not a question of it being "good," it's just what happens; possessing inheritable traits which confer advantages in producing and/or raising healthy, well-protected, resilient and adaptable offspring means a greater likelihood that your line will 'live long and prosper.' While this observation doesn't help answer a question like "Why is cheating bad?", it
is relevant to why we're capable of contemplating such questions in the first place.
(I know some people believe in relativism, but for morality to have any real meaning it must have some sort of appeal to the absolute)
I think the way I would put it is that for moral systems to be effective, they must be backed by broad social and cultural consensus. No system is ever going to enjoy exhaustive consensus on all points (*coughcough* abortion, gay marriage, capital punishment *coughcough*), and anyone tracking the history of a given cultural sphere over long periods of time will notice major shifts in moral thinking; but at the same time there's tremendous incentive, almost an inevitability even, for us to persist in perceiving, analyzing and responding to our social environment roughly in accord with the general outlines of the moral worldview we were raised with. There's not really any one critical item of belief a moral system's influence depends on, without which it'd suddenly shatter into a million pieces and blow away; we depend too much on, and are ourselves shaped too much by, the guiding metaphors and narratives embedded in it and in the culture more generally for that to happen (which is why Kantian fantasies of an ethics based on 'pure practical reason' alone are ultimately just as improbable, albeit just as 'appealing', as God reaching down from the sky and handing stone tablets to Moses).
Further on the distinction between morality and altruism is that, people often appeal to a standard of behavior. Having a standard of behavior indicates that this is not something that is always followed, but something that someone can choose to follow or not. Morality tells people that they ought to act in a certain way, while altruism would be simply an action someone does out of instinct. Certainly not everyone acts with empathy all the time. How is it that it is so easy for some to ignore this instinct if it is so prevalent to our survival?
An instinctive basis for altruism doesn't mean we'll always act altruistically, though; we have the capacity and inclination to do so, but we also have competitive, aggressive and self-protective instincts to contend with. The unique reasoning and communicative capacities humans possess allow us to articulate complex moral codes far beyond the 'proto-morality' displayed by e.g. apes, but the same altruistic and empathetic capacities that enabled our hominid ancestors to flourish through cooperation, in tandem with our reasoning capacities, provide a necessary foundation for the development and transmission of those moral systems. The rhesus monkeys I mentioned earlier wouldn't be able to articulate
why 'I shouldn't hurt that other monkey merely to get a snack'; they wouldn't be able to extract a principle from that experience and ponder its applicability to other types of situations; but we can see from that experiment that at least in this particular instance the 'brakes' built into their basic survival instincts (food acquisition) look very similar to ours. If we didn't possess this same capacity, all the "How do you think your brother feels when you hit him? Would you like it if he hit you? Please don't do that; use words if you want something" in the world wouldn't amount to anything.