Because killing farm animals is considered a norm in almost all places who had done that since generations.
If you would be surrounded by chickens, livestock and other farm animals and watching them getting butchered was your daily bread as a child, why would you reconsider when you would grow up? It literally put food on your table, and besides home grown farm animals taste A LOT better (speaking from experience, but never grown them myself, any meat product I buy from the Walmart alikes tastes much worse).
Its only when you never witnessed death of any kind that you become so sensitive to it. Once it becomes normal (of course, I talk about farm animals here only) it's really hard to see advocates for vegetarianism as some sort of zealots.
And personally? What's the difference if they die of natural causes vs. just butchering them when the time is right? If you raised them ethically and they were generally happy, then for these animals its just... How it would end anyway? If you compare that to industrial farming, and if it was the only way to get meat sure I would consider being vege instead, its horrible how they are kept and treated. But for my own raised animals? Its a cycle of life.