Why aren’t there any baby planes?

Thanks to Shasta-Bob for this joke:

A mother and her son were flying Southwest Airlines from Kansas City to Chicago. The son turned to his mother and asked, “If big dogs have baby dogs and big cats have baby cats, why don’t big planes have baby planes?”

The mother (who couldn’t think of an answer) told her son to ask the stewardess. So the boy asked the stewardess, “If big dogs have

baby dogs and big cats have baby cats, why don’t big planes have baby planes?”

The stewardess responded, “Did your mother tell you to ask me?” The boy admitted that this was the case.”Well, then, tell your mother that

there are no baby planes because Southwest always pulls out on time — now let your mother explain that to you.”