Given how moist the Deep South can be in the rainy seasons of spring and summer time (particularly the line from New Orleans to Savannah), a land owner would benefit from dry roads, especially those leading into his plantation. A large oak tree can transpire as much as 40,000 gallons of water per year, and in one day alone, a large tree can lift up to 100 gallons of water out of the local soil. Planting thirsty trees along a driveway would assure a drier route than other growers with no oaks.

Napoleon installed thousands of Platanus along French roads to enable his soldiers to march in the shade, but I recall a grad school lecturer stating that he did the same along roads leading into Russia to soak up snow melt so that horse-drawn artillery did not get bogged down. I can't find a source for that atm, but it sounds plausible.

This is not a history question, it's a botany question. Southern live oak (Quercus virginiana) grows at a medium pace, and would have been impressive within only a few years and to the stereotypical enormous size with a huge canopy within several decades. The growth rate when young is rapid (7 meters or more in height within the first decade) but after about 50 years or so tapers off markedly, a tree that is only a few decades old may not be significantly shorter than one that is 300 or even 1500 years old.

In addition to their aesthetic properties, live oaks had commercial purposes. They were key materials for shipbuilding, to the point that the Federal government actually set aside land for their cultivation. The trees grow in curves useful for constructing certain parts of ships. Early planters on the Georgia Coast -- John Morel on Ossabaw in late 1700s and Thomas Spalding on Sapelo in the early 1800s -- were quick to set up lumbering operations. The stately rows along drives were decorative, not necessarily for sale, but the trees were a known agricultural product that was already in use, thus familiar and readily available. I know for a fact that the main road on Ossabaw is still lined with live oaks (or was when I was there a few years back).

In the South during that time they didn't have a whole lot of choices in indigenous trees that carried any kind of presence. None of the wet loving trees, such as Gordonias, Taxodiums or Magnolia virginiana have any spread to them at all. In fact, with the occasional exception of Magnolia grandiflora all trees indigenous to the South are singe leader dominant and completely unsuitable, not to mention relatively short lived with quite a bit of that life span being noticeably aged. That is except Quercus virginiana, nothing else comes close. This is still very obvious today in the Southern US, although now we've brought in many other species that have similar growth habits.