Actually, that would drastically change the balance of the game. I doubt they did it because of RAM-constraints.
In Grounded they talk about how they balanced rate of fire, accuracy, damage etc. pretty extensively (and on an encounter by encounter basis). If they had a set number of bullets, we'd have a situation where either there wouldn't be bullets flying while you're in cover (but lots hitting you as soon as you're out), or seriously gimping their aim when they've got you in line-of-sight. Either that or we get the situation where all you have to do is wait until their ammo is out and then have a melee fest. I'm sure there is some way around it, but I think there are many pitfalls there.
I also really wanted that type of mechanic, but I think it probably impacts the combat system too much to do it easily. I do still want enemies to loot through areas and pick up/carry/drop loot if you don't take em out before they get to it.