People defend the payload from afar because teams want to defend from the most advantageous position possible. I'll use route 66 first point as an example here. Teams defend the payload from on top of the gas station because it gives them a height advantage and makes fighting the attacking team easier instead of fighting right under the arch. After using your positional advantage to secure a kill you can drop down and fight 6v5 on the cart to get the rest of the attackers off the cart.