Nostalgia. Pure and simple.
Everytime I look at my old rides it reminds me of the great times I had riding them. Fast centuries. Suffering in heat and rain and cold. And also of when I was (much) younger.
Besides, what is a 40 year old Raleigh Pro worth? Next to nothing. But a lot to me.
Someday, when we move to another city or downsize, I will have to thin the herd. It will be a sad day.