Nick Alvarez's answer to Vegetarianism: What is an intelligent argument and/or reason for not becoming a vegetarian? - Quora
"First, the question asks about vegetarianism but if one is concerned with the amount of animal suffering that factory farming produces,* then there is no non-arbitrary reason for having a vegetarian rather than a vegan diet: If one is concerned with animal suffering and wants to alter their diet in response to animal suffering, then one ought to be a vegan. Here then is a first criticism for vegetarianism: it is morally inconsistent or morally lazy.
There are deeper criticisms to be levied, though. First, we should ask whether a change to a vegetarian/vegan diet is morally required of us once we know about the plight of animals. Suppose that causing animal suffering is a bad thing, what follows about how we ought to alter our behavior? This depends on whether we take ourselves to have positive or negative duties towards animals.
A positive duty is a duty to act in some way towards another and a negative duty is a duty not to act in some way towards another. If I make a promise to pay you $5, then I have a positive duty to give you that $5. Generally speaking, I have a negative duty not to take your $5.
If we take ourselves to have a positive duty towards animals such that we ought to work to end the conditions that cause them suffering, then vegetarianism faces further difficulties. First, it is not clear at all how eating a vegetarian diet satisfies this positive duty towards animals since it is a purely negative act: the vegetarian is but restricting his/her behavior in certain ways. Second, even if eating a vegetarian diet did satisfy the positive duty, there are more effective means of satisfying it (because individual action in this case is practically useless). For instance, by donating money to animal rights groups. There is no clause in place that says one cannot eat meat and at the same time work towards changing the way that meat is produced. Here is the second criticism of vegetarianism: practically speaking, vegetarianism is inefficacious.
If we are skeptical about the prospects of establishing a positive duty towards animals, though, we may think it better to cash out a concern for animal suffering as a negative one. We might take ourselves to have a negative duty towards animals such that we ought not contribute to the farming processes that cause them to suffer. The problem for vegetarians here is that the production of animal products besides meat seems to produce just as much suffering or even more (since it prolongs their life) than the production of meat. So, buying milk or eggs contributes to suffering just as buying steak or chicken does. We have a third criticism of vegetarianism: vegetarians are not in fact respecting their negative duty towards animals.
So, if vegetarians are morally inconsistent (criticism one) and they are respecting neither a positive nor a negative duty to animals (criticisms two and three, respectively), then why be a vegetarian?
*Though there are many routes to vegetarianism, I will presuppose ethical reasons."