The point isn't realism, Bebpo. Nothing is realistic about talking humanoid turtles. However, the fact they're mutations, born and bred on Earth, is the very core of their characters and one of the main points of the story.
It's like changing Superman's origin story to him being born from a Olympian God. Sure, it doesn't change the fact it's a man who can shoot laser beams out of his eyes, but it changes the entire identity of the character.