It's a way of ending things on a positive note. Guys do this too, by the way. Really, you know you will never see each other again but it's one way of dealing with an uncomfortable situation. There is only one person amongst my friends with whom I had a year long relastionship. We didn't work out as a couple but he has become one of my dearest friends. Guys who I have been on one or two dates with I have never seen again in spite of the "let's be friends" line.