I think also the other reason you can't take everything in the vehicle is because of the available ammunition to start with. If you look at the 5.56mm and 7.62mm slots in the squad before you acquire ammunition you'll see that there is not space to acquire all of those ammunition natures stored on the vehicle. This I'm sure will reflect the number of ammunition pouches that the soldiers have on their webbing/load carrying gear/assault vests.
Encumbrance will also be a factor - do you know how much ammunition weighs? On top of the weight of your helmet, body armour and the water in your Camelbak, carrying 6 x magazines of 5.56, a belt of 200 x 5.56mm link for the SAW or 7.62mm link for the MG, a smoke grenade and a couple of hand grenades soon tires you out.