Because it's supposed to be nothing more than an easter egg. Addionally, if it was flyable in the sim, people would have access to its stats in the game, and I didn't want that, because I wanted to be able to adjust the stats later for UE2 without anyone complaining about inconsistencies. The way it is right now, of course, people can still check its stats by opening up the ship file, but there's nothing to indicate that these stats are official.
(BTW, if you look at my tutorial to EXE editing on KillerWave's site, you should have no difficulty editing UE's EXE file to allow you to fly the Rapier IV in the sim...)