Why does the headset man not get on the tractor?
The "headset man" is a wing walker, and he is there for a reason - he is not just someone who needs a ride to where the aircraft is being pushed to. Essentially, he is an extra pair of eyes for the tow driver. He needs be able to see everything going on around the aircraft to avoid collisions with other aircraft or stationary objects. This task cannot be performed from within the tow truck since the view from there is relatively limited.