Normative truths are just as foundational as descriptive truths. You use the same logic to get there. I hope you're intelligent enough to be an epistemological nihilist, so hopefully you know the basis for all scientific and descriptive understanding of the universe is self-evident axioms. The same is true for moral truths. Harm is axiomatically bad in the same way that our senses are accurately able to translate information of an external universe into our brains.

If you disagree with the former, we can't have moral discussions, and if you disagree with the latter we can't have scientific discussions. This is how the whole of epistemology functions.

You're also strawmanning me. Ought implies can, so an animal without an ability to act morally obviously has no moral obligations. I hope you somehow just severely misunderstand the vegan position, and you're not intentionally spreading misinformation.

Factory farms aren't us allowing them to sort out their own problems. We spawn billions of sentient creatures into torture boxes every year just to slaughter them when they're a few months old in brutal and terrifically painful ways.

If you think that's awesome, keep buying meat, more power to you, you're just probably a psychopath (though I obviously can't give you an official diagnosis).