Because of the conservation of momentum.
It is because the conservation of the momentum.
When you push something in one direction, you are pushed away in the opposite direction to conserve the product of mass times velocity.
If you have two bodies interacting with masses
The effect is that a small mass hitting a big mass will bounce with an high velocity, transferring only a very small velocity to the big mass.
When Earth is hit by a light object, its velocity is affected by the impact but due to the enormous mass the change is negligible.