The IRS is already essentially doing your taxes already (certainly for the most common and straightforward returns): they have to verify your statement. The actual computation is trivial (for them).

"Doing your taxes" mostly involves showing you their calculations. It's almost a freedom of information issue.

Compared to the cost of private prep and filing, it basically does seem free (gratis). And even if you choose to dispute or file independently, such a program could save each filer time and money by providing a base document; you would just provide diffs.