Because anyone with even a basic understanding of ecology recognizes that it is literally impossible to live on this planet without your existence causing the death of another creature. Let me ask you this- which is more horrible, the millions of cattle that are slaughtered for beef or the 70 billion creatures that die each year to produce staples like corn, wheat, rice and soy? At best, you can say they are karmically identical in that both resulted in the death of a creature, but at least in the case of beef, that animal is allowing another to live and isn't rotting in an open field somewhere, forgotten and abstracted away by the supposed ethical purity of vegetarian food.
Animal livestock can be used regeneratively in a way that fosters life. There are no sustainable agricultural systems either man-made or natural that don't involve animal inputs in some way. Either we can align ourselves with the rapacious practices of broad acre modern agriculture culture - the monocultures, the chemical inputs, the destruction of habitat - or we can live closer to closed loop natural systems that create habitat and improve the life conditions of all associated sentient beings.
I'm not saying every meal you eat should involve meat. That isn't sustainable either. But to eschew all animal products is to wholly disconnect yourself from the life web just so you can delude yourself into thinking you are eating a harmless diet.
And I think the Buddha was pretty clear as to what constituted killing and the negative karmic consequences of you personally engaging in that act. No action is pure if you pan out far enough. Every action, no matter how noble, has unintended negative consequences. To be a Buddha isn't to be perfect or to engineer a perfect world. We are all hanging from a tree by our teeth. If we can offer one word of wisdom, we fall to our deaths. If we stay silent we will be killed. Our practice in this world is to find a way out of that predicament.