I know what you mean, but consider two successful franchises: What have there been...five Batman films with four different actors? Christian Bale is by far the best. And how many actors have played Bond?
Hmmm...good point. I guess my biggest frustration with the Bat movies is that they seem to be unable to settle on a leading lady; unlike Superman, who has Lois Lane, and Spider-Man, who has Mary Jane. Batman just can't seem to find one to stick with.
OTOH, maybe that's truer to the character himself--he can't commit to one woman because he's too obsessively committed to his dark double life.
And, bringing up James Bond is valid, too. In fact, with the last movie, I personally would have liked to have seen them do away with consistency and find someone other than Judi Dench as M...nothing against her, but her returning presence to a series reboot kind of seemed strange.