Why are animals food, though -- just because that's how we currently treat them? I think the status quo bias is obvious here. After all, you'd never want people to start farming humans, right? So why agree that it's okay once it starts?

Could your argument have been used to justify slavery?

Yours certainly could. It's a fully general moral counterargument.

You have ethics that aren't mine. You want to keep your ethics and not convert to mine. Other people have different ethics to me and also different ethics to you. You don't approve of those other people keeping their evil ethics because they puppy kicking baby eating status quo loving slavers. Therefore, you should convert to my ethics.

Having a preference and keeping that preference isn't a status quo bias. It's the status quo.

Why are animals food, though

What flavor of 'why?' are you after? The 'flavor' one seems most significant to me! It is also sufficient.

There is no problem with just not having an ethical problem with a behavior simply because you don't currently have any problem with a behavior, like the behavior, do not wish to self modify your ethics and are not persuaded by the ability of someone else to find some similarity between the behavior in question and some other behavior that you do have an ethical problem with.

After all, you'd never want people to start farming humans, right? So why agree that it's okay once it starts?

There are perfectly good circumstances to start farming animals, like when your survival depends on it. I suspect that there could be a similar situation with farming humans (or at least process them into Soylent Green). Other that that, I agree on the status quo bias.

Re slavery:

Yes, this obvious analogy occurred to me. I would feel more urgency to reevaluate my ethical system if I considered farm animals my equals. Your reasons for doing so may differ. Presumably the emancipation was in part based on that reason, in part on compassion or other reasons, I am not an expert in the subject matter.