Eastwood has actually been directing since the early '70s (he even directed Unforgiven, and won the Academy Award for that). But he has had more staying power. I don't know if that's a function of him adapting to the times though, or just being really good at what he does. He is smart, has good taste, is creative, can act in a variety of roles (and always could), and had a compelling take on human behavior. Arnold, he was a great '80s action hero. But that's about it. He never struck me as particularly creative, or to have much range. So when the genre he excelled at died out, and as he got older he lost the one thing that he could really do well, and had to slum it by going into politics. Stallone had a similar story, though he obviously was a tremendous creative force in the late '70s with Rocky. That didn't seem to last though, and he became a bit of a parody of himself in the Rocky and Rambo sequels.
Cruise has a bit of range, to be sure, but in most of the films he is in nowadays, he plays more or less the same character. But he does it so damn well, that he doesn't need to change it up. Not sure why he doesn't age. Probably black magic.