the nice thing it doesn't affect our speed or movement

But if it would be realistic, then it would. A heavy backpack would slow you down. The heavier the backpack is, the slower you go. I do not know the attributes of the backpack in this game (I have not played it so far), but the items you carry in your backpack should have a weight and space value. The weapon you hold should also affect your speed. Of course I have not tried this game so far, so I currently do not know how much realism is in it.