[t-3]

Taxes aren't fun or sexy. The users will invariably include a lot of "non-technical" people, which is not going to be worth dealing with for free. Some of them might even try to sue you if there's a bug. A lot of open source software is written by students, retirees, unemployed people, who don't always work or care much about taxes. Pretty much every country has different rules, and I'm sure the US is not unique in having a multilayered system of federal/state and possibly city taxes, so making something widely usable would require a ton of work from people in many regions. Tax rules can also change from year-to-year, placing additional burdens on unpaid maintainers and contributers. Honestly, this is one area where paid software is the best solution if the government is not willing to write their own.