they would not sell many if they honestly would advertise them as
"our ultimate all-in-one , turbocharged
introducing the xx month old iMac with already xx month old processor architecture and xx month old graphicscards , we know there are better graphic cards out there but with higher power consumption and because we care about your electricity bill we decided against them , by the way the PSU inside would not even be able to handle the higher power consumption and the case and fans could not handle the heat of a better graphicscard "
would you buy something if you read a advert like that? because there are much better graphics cards on the market , but desktop graphics cards which can draw easy up to 200watt and more , so not suitable for a AIO if you want to keep it thin,
not to mention that the case design would not be able to cope with the heat output of such a desktop graphics card, therefore you get the newest and best suitable for the iMac, and they will always refer to the newest iMac on the market as the newest iMac
ok there had been exceptions in the past when apple dit "silent" upgrades , last seen on the Mac mini G4 1.5ghz to my knowledge which was never officially sold , i had one of them in my hands , even the box still said 1.42 ghz so did the mini itself underneath , but in realty had 1.5ghz and a ram upgrade of the graphics from 32mb to 64mb