The answer: Because the alternative would be worse.
Imagine if there were no solid background between the text and the background image. You would end up with text against an unpredictable background, which doesn't help readability.
Take the default background for Windows XP: There are some places that are very light and other places that are very dark. No matter what color you pick for the icon text, it will look bad in one or the other place. If you decide to use white, then the text becomes unreadable in the clouds. If you decide to use black, then the text becomes unreadable in the shadows.
You lose either way.
The solution is to intercede a contrasting color to ensure that the text is readable. If your video card is powerful enough, the contrasting color is used only just around the strokes of text themselves, which lends a shadow-effect. If shadows are not enabled, then a solid block of contrast is used.
(And for those of you who say, "Use white in the dark places and black in the light places," what if there is a section of the wallpaper that has a dark area right next to a light area, and the text covers both?)