Why can we not use our own rifle/pistol in battle?

It wouldn't make sense. You bring your 3k AR to the fight and something goes wrong with it, there are no custom shops to repair it, or replace items that get damaged. That 2K ACOG you bought on your own, gets damaged, you have no one to blame but you. Then you'll have those high speed individuals who want to use irregular calibers that NO ONE in any theater uses, could be unlawful, and how would they get more? You cannot send them through the mail. It would be a disaster in the making allowing that. I knew of one Soldier from Riley who got permission to carry a pump with him. It went up the chain but the weapon was a common shotgun with no butt-stock and he knew the chances of bringing it back were slim, but he knew he could get ammo for it in country. And let's not forget Carl who had all this deployment money and decides he wants to bring his Barrett over there. There is no possible way anything could go wrong with this, now is there?

The simple answer is "Logistics."

We have to be able to support what is out there. The more things that are out there, the more things we have to support.

When talking about weapons, that spirals out of control VERY quickly. Caliber of Ammunition, Magazines, Parts, Accessories, and repair capabilities.

Supplying your own weapon is juts not supportable nor sustainable when you are talking about 1M people.

Too many variables, not practical.