I Contracted Myself

A general question for all post-apocalyptic manhua/manhwa:
Why do they always make herbivores mutate into viscous carnivores?(except deer, they love making them into these majestic holy beasts).
And why don’t domesticated animals mutate, like your pet rat will be fine but sewer rats will mutate and cause a rat-tide(and you cant even say its a city thing coz farmland is the closest thing to the wild that humans have and is filled with natural energies yet you never here of mutated cows, chickens, sheep, dogs etc)
Also, when every single and multi celled organism is mutating why aren’t more plants mutating. You’d imagine in those stories were the world is filled with mana and every wild beast imaginable has evolved/mutated you’d have whole forests of magical plants appearing. Farms would grow mana infused crops, the cows and sheep would be infused with some small amount of mana even without a bloodline talent thanks to the plants they eat, especially in those stories where civilization somehow managed to survive/rebuild.
Like, what is happening to everything else that isn’t a player or a normal human in these worlds were suddenly mana is everywhere.
Clearly wild animals and plants don’t need to awaken or be taught how to wield mana they can just naturally do it which means animals as a whole can wield mana instinctively but humans and domesticated animals(whilst also being animals and having the same biological roots) cant’?