That would make sense, except for the fact that certain quotes have a tendency to get used for so long, that no one remembers where its from. Again, as I stated in an earlier post, my favourite example of this is: "Abandon hope all ye who enter here" which has been re-used and re-worded and re-stated MANY times in MANY places, but it originated in Dante's Inferno. But the question I posed the first time I mentioned this is- how many people know that? It is a famous quote, no doubt, but you may not neccessarily think Dante, when you hear this quote. Just like for the longest time, I didn't associate "Be afraid, be VERY afraid" with Cronenberg's "The Fly", because I had no idea that was where it was from (but the movie buffs all know it)
So I think it is actually possible that we all find it familiar, but can't pin down why. This quote has always been for the movie geeks, and I think that may have hindered this search a little- yes, the question has permeated itself to many corners of the internet, but mostly it has been posed to the movie geeks.
So- what if it's not from a movie? What if it's a historic quote? What if it's from a play or a book? How many art-literature finatics or history finatics will have seen or will see this question online? We have to eventually resign to the fact that this quote doesn't exist, but I think we first have to admit the possibility it might not be from the medium we think it is.
After all, if dozens of people can imagine Danny DeVito saying this quote in a Simpsons episode when HE NEVER SAID IT, then it's possible that since everyone is assuming its from a movie, their minds are creating a context for it. (since everyone says "I can hear so-and-so saying it, etc)
That being said, I've spent countless hours on Bartleby.com and freebooks5000.com, which little success, since they don't come with search engines that work well with this sort of search.