Why Don't Big Planes Have Little Planes?

A mother and her young son were flying Delta Air Lines from New York to Detroit. The son turned from the window, looked at his mother and asked her, "If big dogs have baby dogs and big cats have baby cats, why don't big planes have baby planes?" 

"Well," the mother said, "Maybe that's something you could ask the stewardess." 

So the boy asks 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 indeed the case. "Well then, tell your mother that there are no baby planes because Delta always pulls out on time. You can ask your mother to explain it to you."