Frisk also can't move his body against magnitudes more force than Sans is outputting.
Sans can in fact target Frieza, and he can ragdoll him via his soul, but the soul is contained in the body and Frieza can also fly and thus change his momentum and trajectory, should his flight surpass the TK. Unless Sans literally soul rips, the soul would be bound to the body, and at that point we have to ask, what's stronger, Frieza moving his body that contains the soul, or Sans moving the soul that's housed in the body?