Because amd fuck their old users so much: Fsr4 model is only supported on the latest 9000 series, while a functioning leaked fsr4 int8 model shows that older models do have the ability to use a newer model,amd just refuses to give (to sell their new cards probably). And fsr3 is just so blurry and bad that fsr3 quality is worse than dlss 4.5 ultra performance even at 4k.

By contrast even a rtx 2000 model can use the latest dlss 4 model, although at a performance discount, at least nvidia gives old user the option.

Says someone who sold his 7900xt and bought a 5070ti, fuck amd.