Fairness is something that takes intelligence to recognize, understand, and arrange. One reason the world isn't fairer is simply stupidity.
"Might makes right" is an easy rule for animals to understand. If a lifeform is simple enough, it'll never appreciate the difference between fairness and unfairness anyway—or may experience nothing at all.
It's because we have conscious experience, and because we can become aware of fairness, that it can matter to us so much.
I say this because while I believe fairness is very important, I think if you are getting depressed about the incredible unfairness of the animal kingdom, you can at least consider that many of the animals do not process as deeply as we do and may not be keenly aware, much if any of the time, of what they are missing.
