Board Thread:Questions and Answers/@comment-26889090-20190125201705/@comment-26186871-20190126020939

Probably because Mario and Luigi had one (The ending of Yoshi's [New] Island).

But yes. It was by storks-That's how it works in the Mushroom Kingdom.

Really, though, I think a lot of them do this so they can introduce their OC.

Or, it was because of a comment by Miyamoto. When asked about the relationship between the Koopalings and Bowser are, he stated Bowser Jr. was the only son of Bowser, and that "we don't know who the mother is."

But given that they were also explicitly stated to be Bowser's kids in the Super Mario Bros. 3 manual, and he later "clarified" that HE was Bowser Jr's mother (when asked said question in a myth-busting video:https://www.youtube.com/watch?v=uu2DnTd3dEo), his credibility isn't looking very strong.