In our case, convoy implies hauling goods overland for supply and resupply (avoiding the issues of combat patrols and the like).
It is simply not possible to move the quantity of goods required by air. Not for the number of people involved, not for the distribution of bases, and certainly not with the terrain. This is not the Berlin Blockade (which lasted almost a year, 200,000 flights and seven participating Air Forces) but the reality of a long, protracted ground operation that needs beans and bullets. And water. Funny thing, US forces like water for drinking and bathing and cooking and …
Anyway some things, those which have time sensitive materials or expiration dates come in by air – some medications, blood products, people, and mail. Cargo mostly comes in via Conex. Over water to Pakistan, then convoys of trucks from there which explains why it both can take forever and that events in Pakistan affect food on the table in Bagram and Kabul. Fuel as well comes in over land; risk and bulk prohibit any other method of movement and this is a landlocked country. Then there are those things which go missing. In spite of RF tags on containers, there are some which have just gone missing and others which have arrived empty. Current rules prohibit shipping of uniforms in any container going in or out by land (nothing like providing an enemy clothing to help make their job easier). I am not sure what happens to the 110V electric guitars which have also gone missing. Rock band in the rocks? Or just trashed as a symbol of infidel culture?
There are soldiers here on base who spend their entire tour on the road, escorting convoys between here and Kandahar. Others located on other bases with the same duty drive other routes; putting their lives on the line every day so that food and toys get distributed. Those routes are high risk/high value for insurgents, Taliban and criminals. Because of the mountains, alternate routes are often limited or non-existent.
when you think about the number of soldiers, civilians and contractors in theater as being a small city scattered in groups around a good size country with centrally located mountains, it is pretty amazing how well the system works.