Because the cart doesn’t belong in the road or parking lot the same way your plate doesn’t belong upturned on the floor of the restaurant. I’m not sure about you but I will even stack my plates and dishes and silverware so the bus boy can grab it all in one go. Maybe that makes me some sort of a holy man.
BTW, a lot of bussers prefer you not do that. They have their own system for stacking things in their trays, and they have to un-stack your things to do it their way. Like, they generally can’t pick up your whole tidy stack, set it in their tub, and walk away like that.
My acquaintances in food service tell me they appreciate the thought, but rather you not go through the effort.
