Because milkmen don't exists anymore - at least not in the U.S. or Canada. Houses were built with little doors from the outside into a chillbox for the milkman to directly place the milk inside.

They're not completely gone. Here in Utah the Winder Dairy company still does milk and other dairy product delivery to the home. Maybe it's not exactly the same with a little door and whatnot but there's still a person delivering milk (among other things.)

Well they exist here in the UK. I've not seen those built-in chill-boxes though - I suppose here the temperature's never been a reason not to just leave them on the doorstep!

You can still get milk delivered in the Denver metro area in Colorado from Royal Crest dairy.

Milkmen still deliver milk in the US, just not in all areas.