[randallsquared]

The usual argument is that taxes are already paying for the collection of data and calculation of amount, so why can't we just use the figure already calculated by default? This is most true for W2 employees without any uncommon circumstances, but there would seem to be a lot of people covered under that.

[the_snooze]

It's a political challenge, not a technical one. There are constituencies that reap concentrated benefits from the current system (e.g., tax-filing services) while imposing disperse costs on everyone else. Also, there are those who believe that the IRS is out to get them, so filing your own taxes is more trustworthy than going with a government-issued pre-filled default. And that going through the motions makes the pain of paying taxes more salient, so you're more likely to complain about it.

If you look at it as a practical or technical challenge, you're addressing the wrong question.