Do they not have [nearly] all that information already? How else can the IRS verify your filing is accurate without essentially doing your tax calculations automatically?
Most people's significant sources of income are already reported to the IRS.
It isn't the income part, its the deductions. They don't know, until I tell them what I give at various places. If I don't want to report the money I give to a charity / church, I don't have to.
Most people's significant sources of income are already reported to the IRS.