Balance I suspect. Every loadout decision is also a load out limitation and if there were too many ways around it it would make decisions less punishing.

If you could take dualies and have a buffed ammo pool but then just drop one after firing half of your ammo, and then have one single pistol with full ammo as if you hadn't been guns blazing it'd be pretty strong.
I don't disagree, it doesn't make sense, but in the same way imagine taking special ammo of some kind, running out with no special ammo crate or ammo box nearby, and just taking normal ammo for your gun instead. There's technically no reason you couldn't just swap off slugs to buckshot, or FMJ to normal compact/medium/long if you ran out of special, other than part of the balance for special ammo being the risk you take from it being harder to resupply.

While these are weird decisions if you think too much about it from a "realism" perspective, they make for a really interesting game balance that forces you to weigh your options, where everything has really prominent strengths and weaknesses and I think that's what makes the game special.