Real problem is that the rules change every year so the software has to be constantly updated and that sometimes requires insider information about what changes are coming. Often the IRS publications aren't available until the last minute or later, definitely not in enough time for proper quality processes. So, while simple returns can sometimes be done with software like this, a lot of people rely on the software or agencies to know all the new rules.

That being said, I would like it a lot if there was a way to file very single form, but fill it out manually in the software, without calculations being done by the software. At least then you could file electronically regardless of what complex forms you need to file with complex worksheets and sub-forms, if the software didn't need to know about those things, just the forms you actually file. As it is, the only way to file these is with expensive software or on paper which can take many months for the IRS to process and you could be on the hook for interest if you file something wrong and the IRS doesn't reject in time for you to correct it and resubmit before interest charges accrue.

Also, a lot of IRS processes require the software to be certified (or at least did the last time I looked at it) because their software isn't sophisticated enough to validate the complex forms, so getting that certification might be difficult for FOSS software. I'll be interested to see how that plays out.

What's wrong with CC0?

IIRC all US government projects HAVE to be CC0 if released to the public. I think the idea is it was created with public money so there shouldn't be any restrictions on how the public can use it.

I want to think here that this is more about not knowing about licenses than malice.

They did open it.

If open source developers can now keep the tax code updated, you're all golden