I've often wondered this too.
For those not familiar with the FAA system, you pass a check ride (flight test) with an FAA examiner and he (or she) then issues you with a temporary licence valid for 120 days which you can use IMMEDIATELY until the proper ones arrives in the post some weeks later. THEY then send off your old licence (if you already held one) and all the necessary paperwork to the FAA having checked it through.
It is a most excellent system which seems to me to work very well indeed.
I'd guess that the CAA are probably worried about examiners accidentally issuing licences / ratings etc. without the proper experience requirements etc. having been met by the applicant, but my view is that if an examiner is deemed qualified enough to assess my flying ability - they should also be qualified enough to check my documents.
Does anyone on here work for the CAA know why we couldn't do this here?