Because gravity is acting on everything in the universe; it's not possible to shield yourself from gravity by having another object between you and something creating a gravitational field. The ground is not falling inward due to the action-reaction force couplet that you just observed, but that doesn't affect the gravitational effect between the Earth and you. If you are standing on the ground, then the downward force on the ground is the weight of the ground plus your weight, but the next layer down pushes back with the same force, all of the way to the center of the Earth.

Here's another example: suppose that you put a book on a scale - the scale will record the weight of the book, because the book is pushing down on the scale.Now, if you put a second book on top of the first book, the second book does not exert any force on the scale because it's not resting on the scale; it's resting on the first book. The first book, however, is pushing down on the scale not only with its own weight but also because it is itself being pushed down onto the scale by the weight of the second book that is pushing down on it. Thus, the scale measures the weight of both books, even though it is only the bottom book that is actually on the scale. If you add a third book on top of the second, the process justcontinues.

There is no net force in the case when you stand on the surface. Gravity exerts a force downward that is exactly balanced by the force the ground exerts on your feet! Hence the net force is zero and there is not acceleration of you in this frame... if you could turn off gravity, your feet would no longer stay on the ground. Then you would remain motionless...again no force would be acting on you....But your feet would not be held up by the ground pushing back against gravity.

You are being held by two different forces which cancel each other out. Only one of them is gravity. The other is electromagnetism (or electrostatic repulsion), which is the force that keeps atoms apart. If you wanted to use gravity to cancel gravity, you would have to have another large body above you which was pulling you upward with the same strength as the Earth was pulling you downward. There are actually several places in the solar system where this works, for example the L1 point between the Earth and Moon --but none of them is on a planet's surface.