Because you want excellent results, that's why. But if you are a graphic artist, photographer, or serious hobbyist, you owe it to yourself to read our review of the Epson Expression 1680 Pro FireWire. Good scanning starts with resolution. The cheap scanners have limited mechanical resolution, but they interpolate like crazy to give an effective resolution that's much higher. You pay a price, however, in edge definition, speed, and other factors.
Inexpensive scanners have limited color depth, too.
Cheap scanners offload as much as they can to the processor in your computer, slowing things down by requiring significant postprocessing time. Good scanners do a lot more onboard, before they send the image to your machine. High-end scanners are far more robust and faster mechanically, too. The Epson Expression 1680 is all that, and more. Our reviewer, contributing editor David Stone, has seen it all when it comes to graphics, and he was suitably impressed by the 1680's performance.