Because dogs, cats, and humans don’t have the same nutritional value as other meats.

As most life cycles go; Lifeform A consumes lifeform C, requiring nutrients Z, and produces nutrients X. Lifeform B doesn’t produce nutrients X and consumes lifeform A, producing nutrients Y. Lifeform B dies, gets decomposed by lifeform C that turns nutrients Y into nutrients Z.

If lifeform B consumes another lifeform B, or something similar to lifeform B, then they won’t get nutrients X.

You get the idea.

The problem is that this is a valid argument, a lot of morality ultimately comes down to drawing the line on what you think its ok to kill in order to maintain your survival and comfort.