And this is just dumb. Most of the great classic rock occurred in the 70s. Even a lot of great bands that began in the 60s did some of their best stuff in the 70s--Stones, Pink Floyd, The Who, Allmans, Grateful Dead. Plus you had Skynyrd, Steely Dan, Eagles. I mean, really. It's not even close.
The Stones, Pink Floyd, The Who, Allmans, Grateful Dead all were musically dead (Skynyrd physically) or on life support by the mid-70s. Steely Dan was good but the Eagles always sucked. The only good thing musically was the punk/new wave sound that began in reaction to disco and flourished in the 80s.