We've had free tax-feeling software in Spain for decades. Right now, the government sends you a free draft which you can just confirm or correct and complement. It is a great improvement as I remember my early days having to fight with a huge law manual while dealing with my income taxes.
The thing is they warn you is your sole responsibility to check for errors and fill in the correct data so you still need to know all the rules or hire someone to give you a hand to make sure you do not forget anything or correct whatever is incorrect.
One of the problems is the tons of special taxes, special allowances, different expenses etc, etc. Yes, I understand as an administration, you have to make sure you create a law that is fair for everyone but the fact that almost everyone needs to hire an expert to deal with, or suffer the corresponding "fining" consequences upon making errors, it is not the best way to treat your citizens.
To be fair though, I have to add, that the Spanish administration hires people to help anyone to deal with all the paperwork, so that helps those not having the economic resources to hire a tax advisor.
Sounds like Spanish government is way ahead of many others. Probably IRS can learn from them. You brought up a good point. IRS and and like agencies already have most of the information needed for filing taxes if not all. Having them readily available and asking the tax payers to check if information is correct and/or update would definitely make things simpler.