This works if your tax situation is W-2 income and investment income from mainstream investments. You start to run into issues for anything you have to report, like small business income. There are also privacy concerns. Mortgage interest is tax is tax deductible, but should the lender be obligated to tell the government it has a relationship with you? In the case of an employer, it makes more sense because the government is taking what it's owed. For deductions, it's what you're owed, so the onus can be more on the individual.