People are very sensitive to how the icons look
Windows XP shipped with new icons for many common items in the shell, and Windows Vista will have even more and newer icons. One of the things the designers have to be careful about with drawing these icons is to make sure they don’t actually look like anything. Let me clarify. The folder icon, for example, is not based on any actual physical folder you can buy in the store. If it were a rendition of an actual folder, Windows would become vulnerable to a copyright infringement charge from the manufacturers of that folder. Instead, the manila folder icon is just something the designers made up from their fertile imagination. As another example, Windows XP came with new icons for the drives in your My Computer folder. One company complained that the icon for removable mass storage devices too closely resembled the drive manufactured by their competitor. The designers had to go back and draft a new icon that looked even less like any actual drives out on the market, yet which users would still recognize as a removal mass storage device.
That’s why icons can’t look like anything.