Roads, convoys, and railroads all fall under that painstakingly time consuming description I shared with regards to sculpting GV approaches to a base/target. Each road segment must be carefully considered, planned, mapped, checked, double checked, or you get 100mph convoys running across runways 20 feet off the ground.
They're currently not in the game because numerous changes to objects, terrain engines, and other things broke how they were laid out in existing maps. It takes ungodly amounts of time to go back over a terrain an recreate them ALL from scratch.
Imagine a patchwork quilt. Say you're working on one tiny bead/stitch/seam in one 6 inch square that has to be just right or the quilt is ruined. Now imagine the quilt were a mile wide and a mile long, and any time you had to go to one little seam/stitch/bead you had to not only find where you're working, but keep in mind where it is in relation to the big picture and the total quilt pattern as a whole. Working on AH maps is hard. Don't forget that. It's VERY time consuming.