If you believe one should be ethical and compassionate to others then you have to extend this view as far as possible.
If you don’t, then others can decide to pick and choose who deserves moral regard—which may conflict with your own views on it.
Attempts to make humans the sole beneficiaries of moral worth don’t work because every standard and quality that is used –from reason, to a soul to divine favor to evolution, to survival of the fittest lack objective proof –either all humans/nonhumans cannot be shown to possess or lack the quality, and/or Nature doesn’t alter the laws of physics or weather to cater to the alleged superiority of humans, and humans themselves defy this claim of superiority by preying on each other since the start of recorded history.
Extending moral regard to nonhumans simply closes the loophole that would allow racial supremacists, religious extremists etc to discriminate against other humans using the same kind of discriminatory attitude that an opponent of animal rights holds dear.
In short, you cannot have consistent human rights without rights for nonhumans.
It isn’t about perfection (since being perfectly moral is impossible even when dealing with humans and just as we don’t say concentration camps or mass homicide is justified because of traffic accidents the same is true if talking about the accidental killing of an insect being used to justify slaughterhouses or laboratories).
Humans are the ones who decide they need behavioral control through moral laws—thus they are the ones obligated to follow them. Nonhumans are not obligated since they cannot be expected to honor moral contracts which we know they are not capable of honoring (to punish them for this inability is the same as punishing a blind man for not being able to read a road sign). Nonhumans still benefit from consistency and fairness requirements in human moral beliefs.