Creating the 3d model is only the very tip of the iceberg I would think. Then comes the arduous task of the flight model, the damage model, the armament, the weight, the fuel capacity, the sounds, the engine and other components.... the list goes on I'm sure. A huge amount of time and energy has to go into getting the plane relatively accurate from source material nearly a century old.
I'm not a programmer in any shape or form. I would guess that having random aircraft offered regularly would slow down the addition of new planes that we currently receive rather than get more planes into the game.