Why does the coachman sing?

The other side of this paper is that the title, as I mentioned, is based on a late 19th century Romance of the same title (Когда я на почте служил ямщиком, in Russian).

It's an odd and interesting fact that -- going back into the 18th century -- coachmen (iamshchiki) were often imagined singing. The Academy of Science author Staehlin does it; Radishchev does it; L'vov and Fomin do it; before them Chappe d'Auteroche and other West European authors. (I describe them in an article I published in the Journal of Early Modern History in the Winter of 2007, "The Singing Coachman, or The Road and Russia's Ethnographic Invention in Early Modern Times")

But it's one thing to note a cliche that runs across a variety of sources, which seems (in turn) to anticipate the rich popular culture of "coachmen's songs" (iamshchitskie pesni) of the 19th century.
It's quite another to ask if there's some real-life referent standing behind it.

Anyone have any ideas why the coachmen sing? Was this common to coach cultures around Europe? Or a uniquely Russian convention? Did they actually sing, or is this just an ethnographic phantom?