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.
In practice there are many countries that don't require citizens to file tax returns, and somehow it all works just fine. Optimize for the most common path - standard deductions etc. Anybody who needs something more complicated will have to do some extra work, but why should everybody be forced to do this when it's rote for most?
It is a nice idea but in practice it would just overcomplicate things. Changing up the Internal Revenue Code is a better way forward.