Because life and the rules of the universe are not a human creation. Fairness is a human concept, in which we decide what that means, and it tends to vary decade to decade and culture to culture. Also worth noting, rules are not necessarily fairness (lots of animals in nature have automatic instinctual systems or rules of how they exist, without a consciously thought-out system of defined fairness).

It's a matter of the order of things. Life can't be fair by default because it's not subservient to fairness, it's the other way around. The concept of fairness derives from existence, it doesn't come before it. It takes a great effort to create and sustain large systems of fairness as entropy is constantly acting to undermine our efforts and destroy us (while we simultaneously deal with irrationality, chaos, scarcity, injury, etc etc). On this planet, only humans expend such immense energy attempting to accomplish elaborate systems of fairness, while trying to strive toward even greater fairness (learning from and applying, attempting to improve over time; our charitable systems for example have radically improved over the last few hundred years).

When a dolphin murders a porpoise for fun, or a troop of chimps raids another territory and murders competing chimps, is it fair? Fairness has no role to play, because it's a human concept. Humanity is still subservient to the greater rules and limitations of the universe, which means our best efforts at applying fairness can inherently never be perfect or ideal. Just the fact that we act against an erosion by entropy at all times guarantees that outcome (to say nothing of the inefficiency thrown into the equation by routine imperfect human judgment).