- Greece has lots and lots of coastline
- The interior of Greece is largely mountains
So you'd be crazy to transport anything over land in Greece if you had the option of using water. That said, I'm sure there were some roads.
"It remained cheaper to transport heavy goods in bulk from one end of the Mediterranean to the other than to haul them without a river-way for seventy miles inland."
Source: The Classical World, by Robin Lane Fox http://books.google.com/books?id=nqKpSKq0v6oC&lpg=PP1&dq=%22...
I think it should be something like "was even harder than you would think because..." Sure it was harder before efficient harnesses, but even after efficient harnesses were invented, indeed even in good terrain, horses and carts were very expensive. Note that not only were railroads and trucks a lasting revolution in commerce, they were a radical improvement on another (less enduring) radical improvement before that, extensive artificial canals (very famously the Erie Canal, but there were many others, some successful). Rivers and canals were central to inland commerce in Europe and America because even with good harnesses, enthusiastic breeding of suitable strains of horses, and various other refinements horses and carts are still expensive per ton per mile.
However this http://www.machine-history.com/sites/default/files/images/ho... shows the issue, some of the artwork from Greek times seems to match with part 2. of that image, some not far from part 5. So it was, it seems, seen that the burden needed to be pulled from the horses chest and not from the neck per se.
www1.hollins.edu/faculty/saloweyca/horse/h_tack.htm describes the issue in detail and makes the same claim for the Greeks; again stating that the Chinese developed the horse shoulder harness in about 300 BCE.
An interesting subject, thanks for your citation.