Realistically, HMGs were never meant to be closer than 400 meters from the enemy (from the Maxim manual, IIRC). Of course, they could effectively be used much closer, but since the distances in game rarely exceed 200m, they are already out of their element. The side to side traverse angles are actually about double of what the real mounts would allow (eg. MG34 is 15 degrees, whereas in RO2 it's about 30), which should give you an idea of how far away they are meant to be set up.
From a level design perspective, HMGs also need to be out of spawn protection (ideally) because we want players to be able to actually get up to the gun and take it out. I think most of you would agree that it would be quite cheap to place them far back behind the lines imbedded in spawn protection. The HMGs are also quite difficult to place because of the different mounts (arched base for the Maxim and the tripod for the MG34) and height requirements. Wheels were removed because a) the guns are fixed, so the polys couldn't be justified; and b) they got in the way of trying to place the gun on walls and windows.