That's still pretty surprising. That one has to be among the 100 most famous movie quotes of all times I think.
True, I know that the movie is one of the most well known movies out there, including qoutes from it. And that scene at the end or somehting, with the plane, is pretty damn famous. Same with, lets say, Gone With The Wind, another very big and famous movie. But also another one where I would not be able to qoute a single line from it. Only scene I know of in that one is the kiss scene. *L*
Another reason would be that I have never really been that good with remembering qoutes from movies. Very few select movies that I do remember some from. I can also mention that I compleatly suck at remembering lyrics to songs, even my favorit songs. XD The only song I have always been able to fairly accurat remember the lyrics to is Sweet Dreams.

Reason for that is that it is the song I usualy run through my head a few times when some horrible song gets stuck on my brain. XD