You misunderstand- its not that the players have to keep track of which ammo they have or have used, its that everyone has to be aware of whether an ammo stash they found will work with someone's weapons or not.
In a fantasy world with most ammo just being sorted between arrows and bolts, its not a big deal.
But in the modern era, if you're running low on ammo for your 50cal Desert Eagle and you come upon some clips for 9mm pistols...you'd better grab that ammo and start looking for a 9mm pistol because you might not find any 50cal ammo. Then, when the DM has the next patrol you encounter using 5.62 ammo, you can either get pissed off...or loot the bodies for ammo AND weapons that use that ammo.
See where I'm going with this? Games with firearms generally- but not always- have so many ammo types that it can be a headache for players AND GMs to keep themselves properly equipped with reloads.
