Because there is no non-vegan equivalent, no matter how ethical it is perceived to be. Because veganism doesn’t wait for octopuses to be farmed before raising concern, or for science to prove that "common" animals warrant protection. It doesn’t tie moral worth to IQ tests or problem-solving skills. A being need not open a jar lid to deserve respect. It asks us to step back from the habit of drawing lines: this species counts, that one doesn’t.
To argue that this line must remain is to sanction the exploitation, testing, hunting, and mass slaughter of every sentient being trapped behind it. And in making that judgement, it becomes clear that it is we, not they, who lack the sophistication we claim to prize.