Why do so many US homes have their mail boxes on poles like that? I often see them on films and have seen them first hand in some parts of the US and have never understood the reason. Is it because the postal delivery service will not deliver to the front door?
Yes, basically, you've got it right.
Long ago, all mail was picked up at "Post Offices", located in most communities.
Then, at some point, within the limits of cities and towns, "door-to-door" delivery began, facilitated by "Mail-carriers" who delivered on foot.
But those residing in more rural locations, still picked their mail up, from a "Post Office Box".
At some point, "Rural Federal Delivery" ( RFD ) was instituted, to serve those out-of-the-way locations, ... and to this day, many such addresses are still known as "RFD box numbers".
Those RFD Mail-carriers deliver from vehicles, to a curbside "Mailbox", of the sort in question.
In the post war era, the lines between "town" and "country" became blurred by the growth of "suburbs" and "housing developments".
Delivering mail to single family homes, on individual lots, was too inefficient for door-to-door service, therefore, curbside mailboxes became the norm for the majority of Americans.