Why do kids prefer to spend more time with their dad than their mom?
They already know that their dad is gonna get "Milk" and never return.
Listen up, you smooth-brained goof. The joke is about a stereotype of dads leaving to get milk and never coming back. You see, it's funny because it's an exaggeration of a sad reality and a way to use dark humor to cope with abandonment. The kids in the joke prefer to spend time with their dad because they already know the dad is gonna leave to get the "milk," but not really.