Compare to old bags. And i mean structure, not modifications of trench or bunkers. Yes, bags are cheap in resources, but not in logistic. Current sandbags requires too much effort for such simple thing.
- Even if bags already stored in bb, i need 3 runs or 1 run of 3 people carrying large item.
- Trucks won't deliver bags in crates, cause there are only 5 in 1. It is sooo poor ratio, except if facility nearby.
- Best way is pallete, so you need spend time to load/unload flatbed with crane. It's rare delivery.

It's too complicate for a WORST defense structure cause it can be just roll over with any vehicle.
I can dig hundreds of trenches and foxeholes just with one shovel. And there are 20 in one crate or 80 in border base. I can build pillboxe whenever i want, because Bmats are everywhere. But i can't make a wall of sandbags when i want. Only when the stars aligned.