I can't speak specifically to pack animals, but I can speak to the general concept of "Why don't cheaters ruin cooperation?"

This concept is often referred to as "The Tragedy of the Commons", and is pretty much the same thing as the economic principle of the same name.

I used to study biofilms; a certain kind of bacterial community. Basically, a whole bunch of bacteria get together and build a nasty glob of slimy crap to live in. Scrape your fingernail over your teeth first thing in the morning. That's a biofilm.

In a biofilm, every bacterium contributes a bit to the manufacture of that slimy crap that protects them all. But bacteria evolve fast...so what happens when one guy accidentally (I promise guys, it was an accident) mutates and inactivates a gene that makes that goop? He's still living in the goop that everyone else is making...but he's not contributing (Cue Scumbag Steve meme). Now, he can dedicate the resources to other pursuits, like making copies of himself.

And now you have more guys not contributing, and they grow. And so on...evolution in action, baby. But then at some point, there will be too many "cheaters" to support the "cooperators", and the biofilm will collapse.

But that isn't what we see.

Biofilms are, in fact, very hard to get rid of. You could get a lot of money writing grants to figure out ways to kill these things that logically should be pretty unstable.

One big reason is something called "Simpson's Paradox."

It's complicated, but the end result is that if you just take one biofilm, the cooperators are at a disadvantage and usually outnumbered. But there is variability in the ratio of cooperator:cheater, and the communities with more cooperators will be healthier because they are supporting fewer Scumbag Steves per capita.

So if you look at a dozen communities, the communities with more cooperators grow faster. If the community grows faster, its components grow faster. And because "grow faster" is the bottom line of evolution, any bacterium's fitness is not just judged by its own genetics, but also by the genetics of the bacteria around it. This is distinct from kin selection; the same concept applies to multispecies biofilms. This is proximity selection.

You have to look at it as a group of groups, with the more-cooperating groups growing significantly faster than the more-noncooperating groups. Therefore, overall, you end up with more cooperators than cheaters.

But as any one group ages, the number of cooperators will dwindle. As a result, the fitness of everyone in that group drops. So it becomes a dynamic situation; as new groups bud off, the ones that have fewer cheaters grow faster, and take over the ranges abandoned by older groups that have more cheaters.