Because, if you have completed the above, you would know that a PMDG product is a simulation all in itself!!  They have managed to simulate real-life airliners about as realistically as possible with todays PC technology.  It's not joke...after learning the 737s by PMDG and flying them almost daily in my flight sim, I am confident to say, if I was given the chance to fly a real 737, or more practically a full-motion 737 simulator (I have visited them, played in them, but have not yet had the chance to fly a 737 simulator), I would be able to operate it from start-up to shut-down and get from say SJC to SAN.  It is THAT real!  I have talked to many Southwest Airlines pilots, played around in real 737 cockpits, shown a pilot that I can program a real FMC by actually doing so, had the opportunity to jump seat on a ferry flight giving me a chance to see it all in action before my eyes and I can tell you, I sure as hell know my way around that cockpit and know exactly what the pilot is doing during the flight...all because I have tinkered with a $50 add-on.  That is damn impressive in my opinion.  PMDG got my $55 today, and they damn well deserve it.  I have taken her for one spin, and I absolutely love it.  FS2004 is not the same without PMDG products.  I don't do much else with the sim.  If I'm playing FS2004, I am using a PMDG aircraft.  Therefore to me, PMDG is the name of this sim.