Short Answer: For the same reason why one is able to move a pile of heavy bricks with a light push of a wheelbarrow: leverage.
Long Answer: Although the patient feels only a slight pressure behind their ear while being adjusted, there is adequate force being put into the neck. Using the first vertebra and the weight of the head to gain a biomechanical advantage, the doctor places their own body on a calculated angle to ensure the adjustment is specific to the patient's individual misalignment. The force generated by the doctor is transferred to the patient in such a gentle and precise fashion that many patients are unaware of any intervention whatsoever.
