Question

why we can't observe the motion of earth

Answer 1

The spinning and orbital speeds of Earth stay the same so we do not feel any acceleration or deceleration. You can only feel motion if your speed changes. For example, if you are in a car which is moving at a constant speed on a smooth surface, you will not feel much motion.

Answer 2

The velocity at which the Earth rotates or spins is same as of us.That is why we don't feel any acceleration or deceleration.
We can only feel it when our speed changes.