Here's one reason:
Somebody drove by, hit my car to break its mirror glass, and then hit the car in front of me to knock its driver's side mirror assembly clean off. Insurance... is sure insurance, so I went ahead and ordered a piece of glass for it for $25.01 shipped. You'll note this is a full order of magnitude less than the insurance deductible for the repair, $250.
Anyone know what kind of magic grey goop they use to affix mirrors to cars? I have a few days to think about it.