Eating Meat Is Morally Indefensible

Everyone has moral intuitions. They (probably) come from some combination of our biology, environment, culture, and personal reasoning. Moral philosophy1 is the practice of comparing those intuitions to each other to tease out contradictions and inconsistencies. Once you've done this, you can discard or adjust some of your positions to reduce the inconsistency. The goal is to create a moral framework that is as simple as it can be, but no simpler2.
I'm not a moral philosopher in any possible sense, but I think that the practice I've just described should be done by everyone. We all have a patchwork of moral beliefs, some inherited from parents, some picked up from friends or the media. Teasing out the inconsistencies helps us clarify what is most important to us3, and allows us to discard beliefs that contradict other, more deeply held convictions. Having a flexible, battle tested, and simple moral framework allows us to make good decisions.
I have observed that when I think about bad things happening to people a long way away, I generally don't feel much. I know that children being murdered in a different country is just as bad as children being murdered in my neighborhood, because I can't see any way that distance would change the degree of badness. It just doesn't hit me emotionally as much as it should. It's not that I couldn't get emotional about it- I could. I could watch videos about it, think about it a lot, and I'm sure that eventually I'd have an emotional reaction to it. It's just a lot more work than if the murders were happening nearby. I think emotions are unreliable indicators of whether something is morally wrong or not.
My moral intuition, on the other hand, comes to the same conclusion whether the children are close or distant. It's very wrong. There are unlikely edge cases where killing one child saves some number of others, but the base case is that it's very wrong.
If other people are like me, then we all have a fundamental insensitivity to scope4. Large numbers, great distances, vast stretches of time. We can't really grasp these things. We can't give people on the other side of those great distances the empathy that they deserve. We can't grasp that a million deaths is one million times more tragic than a single death. It doesn't kick us in the gut the way someone dying next to us does. But, in an interesting twist, we've figured out a way to work around that, if we want to. In the immortal words of Jeff, from the show Community-
I can pick up this pencil, tell you it's name is "Steve", and go like this [breaks the penicl], and part of you dies.
-- Jeff
The reason we can do this is because we're imaginative. We've adapted to our scope insensitivity by learning to imagine what other people might be going through. We take it personally. Sometimes the thing we're sympathizing with isn't even alive. This is mostly great! We have this innate flaw that makes it difficult for us to give events the moral weight that they deserve, and we've managed to fix it (sort of) by using another innate feature of humanity- imagination. But in another way, it's not, because we can turn it off.
If you have some motivation to disregard a thousand deaths on the other side of the planet, it's startlingly easy to just stop feeling bad about it. Motivated reasoning is a hell of a drug5. All of a sudden, the situation becomes complicated. There are other considerations. Maybe they brought it on themselves through some chain of actions, and so you're not obligated to feel bad about their suffering or the suffering of their families. Maybe someone from their ethnic group committed some atrocity, so you don't have to care about them. Sure, if someone else brings it up, you'll say you do- of course you care. In a technical way. The same way I can say that a million deaths is roughly one million times as tragic as one.
But I'm not putting forth the effort to give a damn about them in the same way those students gave a damn about a pencil named "Steve".
Why does this matter? What the hell does this have to do with vegetarianism? It matters because I think that animals have moral weight. And I think most people agree with me. If I were walking down the street, and I caught sight of a guy in an alleyway breaking a rock with a hammer, I would... probably wonder why he was doing that. If I were walking down the street and saw a guy in an alleyway beating a dog with a hammer, I would immediately try to stop him, probably by pulling him away or shouting at him. If I saw a guy beating a person with a hammer, I wouldn't bother shouting- I'd just club him from behind immediately. It feels like two of these situations are morally relevant, and one isn't. But you see the spectrum here: rocks < animals < people.
If animals have moral weight, then their suffering and death is wrong. Maybe not quite as wrong as an equivalent number of people suffering and dying, but wrong all the same. So far this year, the United States has slaughtered 17 billion animals. If it's wrong to harm and kill animals, then harming and killing 17 billion of them6 is roughly 17 billion times as wrong. And any amount of moral weight at all, when multiplied by 17 billion, is a lot.
I don't feel a thing.
If killing one animal is wrong and we're killing 17 billion of them and I don't care, then it's probably because I'm not reacting appropriately to the scope of what is happening. So what would I do if I did feel the appropriate amount of horror at that knowledge? Well, I'd do whatever I could to stop it. I could bomb slaughter houses. I could protest. I could do all sorts of things, and many of them would lead to jail time or at least seriously impact my ability to keep a job and provide for my family. At the very least, the very least, I could refuse to participate.
And when I say that it's the least I could do, I really mean it. It didn't require any difficult decisions. I'm not struggling to feed myself in a wilderness. I'm not scrabbling for every bit of energy I can get. I'm a reasonably well-paid professional in the richest country that has ever existed. I can eat out whenever I want, and there is almost always non-meat food that I can eat- more than there has ever been before, actually.
So the question is, do I enjoy a few more kinds of food, and participate in a widespread immoral practice, or accept a mild inconvenience in the hope that my trivial rejection of that practice leads to less death?
It was, and remains, a simple decision for me.
This is a personal definition. I'm not formally educated in any of this, and I'm taking what seems useful and discarding everything that seems pedantic or pointless.
Although the internet has taught me to be suspicious of quotes from Einstein, the saying "everything should be as simple as it can be, but no simpler" was probably a somewhat wittier phrasing of something he actually did say.
"To crush your enemies, to see them driven before you, and to hear the lamentation of their women" - Conan The Barbarian
"...scope insensitivity is a cognitive bias that occurs when the valuation of a problem is not valued with a multiplicative relationship to its size" - Wikipedia
"Individuals tend to favor evidence that coincides with their current beliefs and reject new information that contradicts them, despite contrary evidence" - Wikipedia
This website keeps track of the numbers, based (I think) on USDA statistics.