Why are some mammals that lay eggs classified as mammals?

Monotremes lay eggs, as do reptiles.

Monotremes' limbs go outward (rather than downward) from their main body, which is also true for reptiles.

Monotremes lack a corpus callosum (which placental mammals have), as do reptiles.

Monotremes and reptiles both have cloacas, while placental mammals have separate openings for urination and defecation.

This evidence all shows monotremes to be a link between reptiles and mammals, but we now think that monotremes just evolved from an earlier branching from the mammalian tree of lineage than the marsupials and placental mammals evolved from. Monotremes are not a link between reptiles and mammals.