
Cf. Bernard Shaw, Back to Methuselah (1921), Pt. I : In the Beginning: I hear you say "Why?" Always "Why?" You see things; and you say "Why?" But I dream things that never were; and I say "Why not?"
Books, Brain Droppings (1997)
Variant: Some people dream of things that never were and ask, Why not? Some people have to go to work and don't have time for all that shit.