It matters that we don't need to because that means that the harm that we cause by eating animals (perhaps, depriving them of their lives, subjecting them to torturous conditions) is avoidable. We are responsible for the avoidable harm that we cause.

And what about the animals? Are they excused their eating of other animals because they can't help it? I think that's a weird thing to say. Animals can't be held morally accountable; they simply aren't the kinds of creatures that can be morally blamed for what they do. (Which doesn't mean that we can't be held accountable for what we do to them.) So, it's weird to talk about their being exculpated by the fact that, say, they are carnivores. For animals aren't in need of exculpation: they aren't the kind of creatures that could be blamed or praised in the first place.

I agree with Alex that animals are not morally accountable: they cannot be morally blameworthy (or praiseworthy). What is harder to explain is why we don't have a responsibility to prevent animals from hurting each other. This is an unnatural thought, yet perhaps technically we would have this responsibility, if we could also find an alternative diet for the natural carnivores. But given how awful is our own treatment of animals, we should perhaps concentrate on getting our own act together first.