Here's why:
* Placental mammals develop their young inside the mother's womb, receiving nourishment through a placenta. Examples include humans, dogs, cats, elephants, and whales.
* Pouched mammals (marsupials) give birth to relatively undeveloped young that continue to grow and develop in a pouch on the mother's abdomen. Examples include kangaroos, koalas, and opossums.
These are two distinct categories of mammals, and they are mutually exclusive.