The US tax code isn't created to work like this. Direct file asked many questions about things like dependent children, whether you're blind and whether you have an HSA - all things that are relevant to your taxes that aren't actually available to the government right now.
Secondly, there is the issue of State / Local taxes - the IRS only receives federal tax data making it hard to automatically fill out the whole tax return since efiling products tend to file federal / state taxes together.
additionally, the US has (one of?) the most complex tax systems in the world. In part b/c most of it is carve outs...on behalf of various lobbyist groups / catering to specific voting blocks.
The majority of the population of the US claims the standard deduction and has all their income in the form of W2 or 1099 which is reported to the IRS by the employer. Those people can be served by a return free filing system.
The minority which have more complicated taxes can still file like they do today. But even adding on investment income and housing related deductions the IRS likely has enough information to calculate what is owed.
We shouldn’t let perfect be the enemy of better. I know you weren’t arguing that point but just because the tax code is complex doesn’t mean it is complex for everyone’s situation
Entering your income is easy enough if you're a salaried employee with a W2 form. The time consuming part is searching for and entering deductions. The tax code is ridiculously complex and there are forms for all sorts of deductions.
Even if you're pulling in 100% of your income through a W-2 job and you're sure you're not itemizing, you have to consider excluded income, above-the-line deductions, and refundable and non-refundable credits. Oh, and do you live alone or might you claim a dependent,[1] and on and on and on. People spend their whole lives on this stuff and still don't understand the half of it.
The same reason the US moves more of the worlds money through commercial and investment services hosted here.
Ask anyone in the EU who has lived in one country and earned a paycheck from a different one.
Anyways, give it time, the EU is currently working to make it's tax system more complicated to solve some of the long standing continental issues, and to make the EU system more like the US one.
It is pre-filled with the known incomes so for the best majority of people filling their taxes is a 1 minute exercise.
This also helps, I guess, to have the taxes flow in.