I understand if like, Charizard lays eggs. It's like a reptile. But what about Miltank and Raichu? I see them as mammals. How do they lay eggs?!
Having Pokemon like Miltank or other Pokemon portrayed as a mammal lay eggs means Pokemon has a new Unique selling points becuase if you add bits into Pokemon like egg hatching it keeps the person that is playing the game interested. If a Pokemon just gave birth to a Pokemon at level 1 without it hatching an egg then players would lose intrest after a while.
Also everybody says this so It's how Gamefreak designed the game :P
Also read this if you can be bothered it is a report on this debate here
Hope this helps