Because empathy being applied to other species is an evolutionary bug, not a feature. Bug probably triggered by the luxurious abundance and the accompanying decadence acting on brains a bit too disconnected from the reality of the animal kingdom.
And if you want intellectual justifications: because we're the apex predators and we do what we want when we want; that includes raising and eating beasts that are of no other value to us. The other carnivorous animals wouldn't act in any other way, in our position.
Sometimes there isn't anything one can do to sugarcoat something: this is stupid. For one thing, even if we were to take your bald assertion that empathy is somehow an evolutionary bug at face value, though you've provided no evidence for this or even really any suggestion of what it could mean, it doesn't follow that lacking it is the optimal behavior relative to "evolutionary goals" (which are nothing) or whatever unstated goals you presume we all share.
Like on of the most wonderful things about human beings is that we can reflect upon our behavior, form moral and aesthetic opinions about how we'd like the future to be, and make appropriate adjustments to our behavior to achieve those ends.
In any case, the material circumstances that produced us have absolutely no meaning at all. The essence of the evolutionary explanation is that life really is random, given that, we may just as freely reject evolutionary understandings of what we want from the world as we may accept them. Surely they are of practical value, in the sense that if we want to optimally act on the world we have to understand it, but they impose no necessary moral or aesthetic demands upon us. It is deeply confused to believe that the mere evolutionary circumstances that produced you ought to constrain how you want the future to look.
"Species" is, in and of itself, a man made construct. To define your own morals by an arbitrary distinction isn't just simplistic, it's downright dangerous.
It wasn't long ago that certain people were considered "subhuman", and therefore open to all kinds of abuse. Did you learn nothing from that?
And of course, the same is true for your ridiculous "might makes right" argument. Have you considered making other horribly uneducated arguments, like "it's natural"? Maybe add some "we've always done it that way".
And if you want intellectual justifications: because we're the apex predators and we do what we want when we want; that includes raising and eating beasts that are of no other value to us. The other carnivorous animals wouldn't act in any other way, in our position.