The Armaverse timeline does not necessarily reflect the development of real-life weapons and armory. In ArmA 2, we saw the SCAR-L and SCAR-H become commonplace US Army weapons for Operation Arrowhead, which we know in real life to be false - the US Army still uses the M4A1.

Similarly, in ArmA 3 the new standard for conventional warfare is 6.5x39mm Grendel. This is presumably because body armor has developed to the point where 5.56x45mm and 5.45x39mm are no longer sufficient. Besides, the CSAT known on Altis are Iranian soldiers, not Russians.

EDIT: US Army actually uses the standard M4 carbine.

We already have mods that allow disposable launchers. The vanilla launchers are as-is because it allows for multiple functions for different users' tastes.