Playing devil's advocate, there is a reasoning behind a Flight Review requirement.

For example, if you do the add-on category for Glider, you are not required to pass another knowledge test if you hold Airplane or Helicopter category ratings. Further, there are portions of the practical test which are skipped: Knowledge of Certificates and Documents, Cockpit Management, Airport Operations (except Traffic Patterns), and Navigation (including Airspace) to name a few.

That, plus as was mentioned, an add-on solo endorsement could last forever.

So, if you have not flown in quit a while, and the rules are changing all the time, the only way the FAA can be sure that you are up to snuff on rules and procedures is if you have had a recent Flight Review.

What would work better is if only the ground portion of the Flight Review would be required for the add-on situation because the above missed bits are covered in that part.

The whole Flight Review bit is weird anyway because a strictly CFI-G, who has no knowledge multi-engine Airplanes, could sign you off in your Glider Flight Review, and you could then immediately legally climb into a King Air and fly away, even though you have not touched one in 50 years. Of course we assume here you are already rated in both category and class of the respective aircraft (Glider and AMEL).

Yes, common sense missing all around. Wait...I think Bob Newhart is calling me...