Why do we allow cadets to use CAP aircraft to obtain private pilot licenses, but we don't allow seniors to do the same thing? I'm not talking about instrument or commercial add-ons, but private pilot licenses.

Instruction of senior member student pilots is prohibited unless specifically authorized in writing by the wing commander, region commander and the Chief Operating Officer.  Regarding seniors getting private pilot licenses, this issue has been brought up several times over the years to the CAP leadership. It has always been turned down.  

Senior and cadet members may receive flight instruction from CAP instructors in CAP aircraft under the conditions in CAPR 70-1.