Becasue we don't drive to the firing point - we carry our stuff and walk to the firing point!
Load bearing gear helps a lot in organizing "stuff" and increases efficiency-much easier to keep track of kits.
I dont know why people carry only 3 mags - I bring 5 mags. There is no time to #### around. Hungry and the crews run things fast - they don't move at "leisure" speed like the rest of the ORA. We get off the firing point and hurry down to the butt. If you don't have your load bearing gear, you could easily forget water and gum down at the butt in a hurry.
People who are packing, unpacking....and having their stuff scattered around cut into my lunch time....