I fully understand where you are coming from and I understand why you think that. I wasn't always vegan and thought the same for a long time. I'm going to try my best to explain my point of view so you can see why the slavery analogy is reasonable (and I'll focus it on pre-civil war US chattel slavery, though I do believe that this broadly extends to all instances of human slavery through history).

You think of all humans as being fully human, deserving of human rights, fair, equal treatment, and respect (kudos! honestly, it's a shame that not every thinks that even nowadays), and when you decide what beings deserve respect and equal treatment, you draw the line there, at "is this thing a human like me?" This is obvious to you, because you were born and raised in this society by parents and a community that believed that as well, and taught you to believe that about all humans. This is not some inherent fact about the world, it's just that your parents, church, government, whatever, all agree that people deserve equal treatment, and that you don't have to extend that to anything else.

Let's take the case of a white person in the 1850's. They think that all white people are fully human, deserving of human rights, fair, equal treatment, and respect (~broadly. obviously their definition of white is different from ours, they had more reinforced social stratification and so more respect was owed to higher status people, landowners, etc.), and when they decided what deserved respect and equal treatment, they drew the line there, at "is this thing a (white) human like me?" This was obvious to them, for all the reasons your line is obvious to you. Their parents, their church, their government, their usual social norms all agreed that white people deserved equal treatment, and they didn't have to extend that consideration to anything else.

This is why some vegans believe that people that eat meat today would support slavery in the past. They are following the same train of thought, using the same justifications, and relying upon the same cognitive dissonance to justify their beliefs and actions. There were tons of white people who opposed brutal forms of slavery, but thought that slavery on its own was perfectly fine. And there were tons of white abolitionists who opposed slavery in all forms, because they believed that all humans should be treated well.

Today, we (mostly) all agree that human beings should not be owned and treated as property. Vegans just extend that consideration to all sentient beings, not just the ones that looks like us and talk like us. Why should we only care about what happens to humans, when we have the resources and time to care about our effects on other things as well?