It's a social construct like any, a partition or compartmentalization we put up to allow us to behave certain ways. Some cultures have different outlooks, some individuals have different outlooks.

I think the only logic to glean here, is that we're able to get more resources out of a cow, so it was useful for us to compartmentalize and abstract away farming them as different somehow, so we did.

Neither of any of the above. All of these animals are used and killed for research all the time.

The issue for many of them is the personal connection and memories individuals have developed with one species or the other. Then it does matter more if it is a pig or a dog that you are doing harm to. For that matter I have known researchers who are excessively fond of pigs, chicken or cows ...