Also it requires expertise outside of programming for which there is not much enthusiasm for open source.
The problem is then: are they shielded from liability? If there's a mistake and people sue, they won't have Intuit's lawyer army.
But let's say we have the perfect program. Would people be OK with printing out and mailing their returns? Because you have to be an authorized provider to e-file.
Except for medical bills, mortgage deduction, education expenses, retirement/HSA/etc. accounts and SALT (if that gets reinstated) and maybe a couple of others, probably. Although, frankly, I'm not sure what deductions other than charitable contributions don't get reported to the IRS and I'm similarly not sure what deductible expenses don't already have a paper trail I would care about retaining. But that's where "correcting the math" would include replacing the standard deductions with itemization would come in.
I think requiring a basis for investment income to be made in the acquired year to the IRS is both reasonable and trivial.
The sheer number of changes that the Tax code makes every year would be hard enough to keep up with for an open source project, let alone making integrations with all of the brokerages to make this process less painful.