Here's why:
* Mammals are defined by several key characteristics:
* They have fur or hair.
* They give birth to live young.
* They nurse their young with milk produced by mammary glands.
* Animals that lay eggs belong to a different group called oviparous animals. This includes birds, reptiles, amphibians, fish, and insects.
So, while some animals may share characteristics with mammals, like the platypus which lays eggs but also nurses its young, they are ultimately classified as mammals because they possess all the defining characteristics of mammals.