Things generally not known by the government: deductions & credits you are eligible to take, any changes to your marital status, changes to your address. Often your government cannot find you, has no idea if you're still single/married/widow, and doesn't have the data for credit eligibility. Besides, you're also assuming perfectly accurate reporting of all data elements on income reporting documents by all payers and employers. We don't even have that now.

It is a nice idea but in practice it would just overcomplicate things. Changing up the Internal Revenue Code is a better way forward.