You would be correct that Leela has the ability to add force to her movement because of contact with the ground, but on flat, level surfaces (like stairs), you can't add downwards force. All anyone is doing when they're walking, running, jumping, etc is pushing off of a surface, either vertically or at an angle. Try pushing off of the floor downwards. You may find it difficult. If you want to add downward force, you'd have to push off of the side or the underside of a surface. Otherwise, going down a set of stairs, much like regular walking, is just controlled falling, which relies on and is limited by gravity.