Julian - when I asked that question, the response was "Because it's illegal - only the CAA can issue licences".
Now the question is - are the CAA any different to the DSA (Driving Standards Agency - as they were called when I left the UK 11 years ago). Driving examiners gave out certficates of qualification NOT driving licences.
That certificate entitled the holder to act as a qualified driver until the issue of a full drivers licence, by Swansea.
The FAA examiner doesn't give a licence or rating either, of course - they just issue a "Temporary Airmens Certificate" that entitles the holder to exercise the privileges of the successful flight test for 120 days to allow Oklahoma City the opportunity to decide whether or not the application for the licence or rating is acceptable (if not, the TAC expires and the privilege is lost).
Absolutely no reason. other than old fashioned view of job protectionism and self importance on behalf of the quango.