They are no longer produced, if you don't believe, search. Apple bought a huge stock when new iMacs were introduced. Why would ATI produce old GPUs, especially when demand for new ones is very high?
And by the way, no Mac (excluding Mini with GMA950) has used the same GPU for longer than two generations...
http://www.garysky.net/apple-to-juice-up-the-imac’s-graphics-with-ati-radeon-hd-5750.html
"steady supply of Radeon HD 4850 chips until the end of June"
Why would ATI produce old GPU's? Because Apple can demand them.
Just for kicks to really blow away that last statement of yours, here's a list of GPU's Apple has used again after a refresh of the machine, some of these have gone for three or more refreshes:
Radeon X1600
Radeon HD 2400 XT
Radeon HD 2600 PRO
GeForce 8600M GT
GeForce 9400M
GeForce 9600M GT
GMA 950
GMA X3100