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.
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.