Why aren't any of the cast from previous Terminator movies returning in Terminator4/Salvation?!?!?

well arnold has retired from acting since he became the govenor of california ,but there was buzz on yahoo yesterday that he may make a cameo. as for the others the movies are set in different age time in the life of john connor and sometimes actors don't want to be pigeon holed into a role. linda hamilton's character sarah died of cancer and was addressed in terminator 3 the rise of the machines and the other cyborgs were destroyed in each movie including arnold,so that may be why. james cameron didn't like the scripts and didn't want to continue the franchise w/o arnold.