I totally agree with Jabber.
This has to do with how the forts exactly work along with their costs and the compared effectiveness of entrenchments. The real problem is the prices to sustain for such an investment for CSA while they are not an option for Union who's rolling on the offensive.
2 carts are worth 80$ which is more than the cost of the biggest virginian infantry reinforcement probably much more needed elsewhere. When used to build a fort they don't produce as much as they would produce as a depot and once they are used...you can no longer used those carts to resupply troops at the front.
4 cannons, of any kind, are also much more likely to be needed elsewhere...especially since they increase the level of any entrenchment you might dig them into.
The cherry on the cake is the fact that once you have "shipped" 2 carts and 4 cannons, once and if you have survived in that position for the days it takes the fort to be built, you still need to bring in EXTRA guns outside and especially INSIDE the settlement for the fort to be effective both as defense and as "bombarding-passerbies" factor.
This would be considered outrageous if the fort really had to be an option.
In fact...as you can see, it isn't. And it's sad.
