They didn't (and couldn't) "exist" until scientist discovered that they did and could.
They were, like many, many phenomenon reported by us mere lay people, thought to be the stuff of myth and legend. How could huge waves, unrelated to weather patterns, earthquakes or any other commonly understood mechanism, just -appear-?
"Unpossible!" they cried in their lab-bound dismay.
Anyway, enough snarking at the poor, hapless, brain-bound "searchers for truth".
Rogue Waves:
http://en.wikipedia.org/wiki/Rogue_wave