We've always eaten meat
How long a practice has continued is not a reason to continue it. A great many long-standing practices were abandoned precisely once duration stopped counting as justification.
Eating meat is not some recent fashion but part of what humans have been for as long as there have been humans: our bodies, our cooking, our feasts and our family tables were all shaped around it. A practice that every culture has kept across hundreds of thousands of years has earned a presumption in its favour, because it reflects accumulated experience of how people live and thrive. Calling it wrong means declaring nearly every person who has ever lived a wrongdoer, and that should take far more than a modern argument to overturn.
The historical claim is broadly true. Meat has been part of human diets for a very long time, in varying quantities, and for most of that period there was no realistic alternative in most places.
Neither half of that establishes the conclusion. Duration is not a moral property: slavery, judicial torture and the legal subordination of women were all traditions of considerably greater standing than any particular dietary habit. And the conditions have changed: the argument from necessity, which was once genuinely strong, does not survive the existence of a well-stocked supermarket.
Treating the length of time a practice has continued as a reason to continue it. Age is not a justification; it only tells you the practice is old.
How it applies here: Offers the age of the practice as the reason for continuing it, which at most establishes that it is old.
Moving from a description of how things are to a claim about how they ought to be, without supplying the moral premise that would connect the two. Identified by David Hume.
How it applies here: Moves from a description of what humans have historically done to a claim about what we are now entitled to do, with nothing supplied to bridge the two.