I'm a TurboTax user myself (b/c I almost always procrastinate to the last day/week), but my guess is because tax forms and rules change every year. Maybe the general situation for free filers is easy enough to code for, though? Though one of Turbotax's purported features is that it'll tell you how easy/complex your taxes can or should be, depending on your situation.

Also, one of TurboTax's features is e-filing. Maybe there are free e-file alternatives (as in free, with no upsell or strings-attached whatsoever), but it seems you have to be an "authorized IRS e-file provider" to do this, i.e. there's not a developer API: https://www.irs.gov/filing/e-file-options