A trite and borderline ridiculous notion based on popularity, desire to understand things in as topical and simple a manner as possible, and confirmation bias. To begin, this is a very popular notion. Add on top of that that it's pretty alluring because it allows one to feel a sense of understanding some aspect of the whole of creative work and then top it off with a healthy dose of arrogant "I understand how all of this works". Given that the majority of opinions probably won't diverge (since there will usually be only a few salient choices in any given situation and that any given idea can only be wholly original once, but is likely to be repeated many times), you get an enormous confirmation bias. In reality, there are new ideas, new opinions, new perspectives, and new pieces of information going around all the time.
More concretely, think of a recent event/discovery. Now ask yourself if people have opinions on that event/discovery. They do? Well that would reasonably constitute an original idea. Some ideas and opinions were simply unthinkable or incoherent before events or discoveries made them available. No one had an opinion on the protests in Egypt before the protests in Egypt (maybe they had opinions on potential protests, but not on the protests themselves). Nobody had opinions or thoughts regarding black holes before black holes were discovered. For originality to be dead, there would have to be no new events and no new discoveries, but events are happening and science marches onward.
Hell, if we want to get really low level, a healthy proportion of the sentences you utter every day are extremely unlikely to have ever been uttered before. Ever.