I think they just love what they do and want to keep doing it until they can no longer perform. By that I mean, literally being unable to, rather than having aged or their voices not being the same as they once were. The fans are also blessed that they get to see their favourite artists and bands and they essentially don't care about stuff like that. For example, my favourite band is Duran Duran, all the members are in their 60s, and Simon's voice is possibly even better today than it ever was, and he's the oldest. I personally don't mind it when rock stars keep playing as they get older, though I do agree with you that it if poses a serious threat to their health, it's best to leave it.