To be fair, they haven't done much of note in the last 30 years.

"Ain't nobody gonna bow no more when you sound your gong"
-Spin Doctors

Kidding aside...these ‘artists’, much like athletes really can’t take time off. That’s why The Rolling Stones, Springsteen, Aerosmith et al amaze me with their longevity and quality of shows. ‘Staying in shape’ (for lack of a better term) is the difference between playing ‘The Tracy’ or playing the Meadowlands!