The big unit using a small unit trenches is not the only abstraction. There are others like...where on the 50 miles wide region are the trenches?? always right in the enemy path? how can we know they are facing in the right direction?? Should a entrenchment value depends of the direction of the enemy attack?? Should trenches remain on a region after the units left?? but where exactly on the region?? facing where??

IMHO the answer of all this questions is the same: game rules abstracting the reality to try to represent the flavor of the ACW.
This is a strategical/operational game, where you direct units form Arizona to Virgina, control economics, diplomacy, recruitment... played on 15 days turns and on big areas lots of miles wide.
Its not a tactical game where you can count size, location and orientation of individual unit rifle pits or number of spades per unit, food quantity and quality to regulate the speed of the digging, or number of preexisting farm fences aroudn to use as defensive lines

Look above the little tiny details and into the big picture effect of the rules!

Just my 2 cents!
