Exactly. The BBJ is designed to be an executive jet from the gear up. A reconfigured 737NG simply wouldn't be able to perform like the BBJ. A reconfigured 752 wouldn't be able to perform like a BBJ3.
Why would Boeing sell a BBJ rather than reconfigure a current airliner? For one big reason. $$$$$$$$ They will get a hell of a lot more money selling a BBJ than reconverting a current airliner. For one thing, Boeing may not even get the contract for doing such work. Also, with so few of them ordered and delivered
. At last count, I think Boeing had sold 75+ BBJ/2's. This number of aircraft has far exceeded what Boeing even dreamed of selling. To Boeing, the BBJ has been more successful than they ever dreamed of.
By the way, I STILL want a BBJ and BBJ3, thank you very much!!!!
I'm not a real aeronautical engineer, I just play one on Airliners.net.