If these animals weren't raised for food, they wouldn't exist...
Why does this keep being put forth as an argument in defence of industrial scale farming?
Its quite clear that it would be better they didn't exist than to be here, destined for slaughter and human consumption.
It's not 'quite clear' at all. As a PP explained it, animals live instinctively, in the now, and they don't understand/plan for/care about the future. If you could ask them for their opinions (which you obviously couldn't) as to whether they would have liked to have however many months/years of life or none at all, you simply have no way of knowing that their personal viewpoints would favour no life at all.
Going on the fact that most farm animals (at decent, humane farms) seem to be quite happy and content standing/lying there, eating, rolling in mud, communicating with each other etc. on a daily basis, and don't look in any way stressed, I would guess that, if they had the ability to rationalise it, they would, on balance, choose the opportunity to live for however long they get; but none of us have any way of knowing for certain.
I still don't recall seeing an answer to the PP who asked about the attitudes of the militant vegans to the huge number of small animals and insects who are killed either by pesticides, by getting ripped up by combine harvesters or even by being humanely prevented from getting to the food that they need to survive, but which we have decided that we are going to have and keep for ourselves.
Food production includes death and survival of the fittest/strongest/most intelligent, even if humans don't actually consume any meat. The only way we can personally play no part in this is if we selflessly decide to sit out of eating altogether and starve to death.
I also think it's very disingenuous when some vegans scoff at those who ask "what about the plants and crops?" Just because they don't have brains or nervous systems, they're still living organisms whose lives you are happy to end in order to continue yours.
Plenty of people anthropomorphise animals and assume they actively want to live as long as possible and/or fear not being alive - or alternatively that, had they the ability to understand, they would prefer to never be born at all; all this when we have no evidence at all that they think or care about their own futures (nor do we have any conclusive proof that crops don't feel stress when ripped up/cut down). Why do we value the lives of animals and impute human attributes to them but then have such total disregard for the lives of non-animate organisms? Either we humans prioritise ourselves or alternatively we refuse to kill any living thing for our food.