Because people who typically have clutter "are perfectionists, afraid to throw something away for fear of needing it in the future.
They are sentimentalists, and forgetful, leaving things out in the open so they wouldn't forget them. They [hang] onto old letters, photos and gifts lest the memories escape." [Orderly House, Orderly Mind - New York Times]
Oh, yeah. This is me to a "t" -- although I'm trying very hard to get better. I now actually have hanging file folders for the most important people in my life, so when I find a stray letter or card from them, it has a place to go. And I have to say that I'm very, very glad I was sentimental enough to keep most of my maternal grandmother's letters over the years; I only wish I had things like that from my paternal grandmother. I agree with organizer Peter Walsh's assertion that if something is important to you, it should be displayed or stored in a way that keeps it intact and usable.