The point is that I might report something that they aren't aware of. It makes sense to me, the IRS does not know everything and strategically witholding what you know will lead people to report things that you don't know about.
The majority of the population doesn't know anything about their taxes that the IRS doesn't. Why force everybody through a song and dance that only is of service to the exceptions (and a couple very large tax filing corporations who lobby hard against automatic taxes)?