.... Yeah, they do... You assume that
A. Planets are held in their orbits with no way to move out of them, like a track line... and
B. There are outside forces to absorb the energy of the collision, causing the force of the impact to diffuse elsewhere...
I can easily say that neither of these exist on the scale that we are referring to... so QFT if you haven't had an actual astronomy class (college level or higher) in the last decade...