Airsoft guns were first created out of the demand for replica toy guns which look and feel (and work) like the real guns. However, the early models made use of plastic and other materials which were much lighter than those used for real guns. That aside from the fact that airsoft guns have different parts from that of a real gun, made them much lighter than real guns. Among other things, the pellets used are made of plastic, and there are no shell casings.
There have been steps made to make the airsoft guns look and feel more authentic than before. One solution is to make metal casings for the plastic pellets. Another is to use metal in the manufacture of the guns. In the pursuit of authenticity, there are metal airsoft guns which are designed to be as heavy as the real guns. These also fire the same number of shots (or pellets) as the real guns.
These improvements of modifications have been followed mainly because some enthusiasts would like to have a more authentic feel for the military simulation.