Because those guys are eternally optimistic. Listen to their interviews. Watch those old film clips. And obviously listen to that music. Music of possibility, of can-do, of We Can Work It Out. They must have been exhausted, nervous and at times fearful but they never spoilt it for their audience.
That's another reason we love The Beatles.
Reason 1: Story-tellers....
Reason 2: Sgt. Pepper