Ergo, imagine there's a checkbox option that would apply to 25% of free filers to save them money. How would the IRS free file handle that from a UX perspective? What are their incentives?
Alternatively, suppose a feature used by 10% of filers is broken for the current tax year. How long does it take to get fixed?
Current private solutions are incentivized to optimize those ("We save you the most money!") and fix them ("Our software isn't broken").
And because only the data transfers over to the IRS, all the prep optimization can be done independent of the IRS.
Some of the incentive concerns would be alleviated by (as this did) building this from a different branch of government (e.g. US-DS).
But at the end of the day, it's still the government telling you how much to pay the government. Or as the joke went about the 1040-EZ: "How much money did you make? Good. Give it to us."
It’s literally data entry.
Tax prep/accounting (what to put where for cases where there is ambiguity) can always be different.
That doesn’t apply to W2 earners though (in 99% of cases)