Because then you evolve societies that spend so much time fighting each other for marginal increases in individual safety that the benefit of working together is lost.
To be clear, though, there's no evolutionary function that optimizes for the overall benefit of unrelated individuals. So-called group selection has never been shown to exist in nature.
Cheating is moderated by the success rate of cheaters and non-cheaters individually. If cheating strategies come easy but counter-cheating strategies difficult, then the species success in the larger ecosystem will indeed be limited in as much as organized, goal-oriented behavior is beneficial. There's no group-wide selective pressure to favor the non-cheaters over cheaters as there's no group-wide inheritance mechanism that can accomplish that.
It's why schools of fish aren't as efficient as one might naively think they could be at evading, e.g., a giant whale. The cheaters can and do drag down the whole group because the equilibrium rate of cheaters in the school isn't responsive to the success rate of schools as a whole.
Cheating is moderated by the success rate of cheaters and non-cheaters individually. If cheating strategies come easy but counter-cheating strategies difficult, then the species success in the larger ecosystem will indeed be limited in as much as organized, goal-oriented behavior is beneficial. There's no group-wide selective pressure to favor the non-cheaters over cheaters as there's no group-wide inheritance mechanism that can accomplish that.
It's why schools of fish aren't as efficient as one might naively think they could be at evading, e.g., a giant whale. The cheaters can and do drag down the whole group because the equilibrium rate of cheaters in the school isn't responsive to the success rate of schools as a whole.