For one, most of these warranties don't guarantee on-site service.
Computer components lose their value very quickly.
For the same money today, you can get a 20-gigabyte or larger hard drive. If you bought an extended warranty and your hard drive failed, the company would replace it with another 14-gigabyte hard drive. For the same money you paid for the extended warranty, you could get a much larger and faster hard drive. Because most people would rather upgrade than maintain the status quo, I recommend against extended warranties. The exception is laptops.