On that note, the following exchange from a tax Q&A on MSNBC says a lot:

Each year, we send in W-2’s and 1099’s along with our 1040’s. I know the information on the 1099’s is already reported, and I believe the same is true of the W-2’s by the companies that issued them to us. Why doesn’t the IRS just enter what they already have in a mainframe and send us out a bill or refund like any business would?
– Matt R., Jefferson City, Mo.

In short, because the tax laws are made by the U.S. Congress, which doesn