All the IRS does is publish the tax code in a specific XML format. MEF (mechanized e-file) is the interchange specification. It's rather a shame there's not more open source effort around tax filings...
This is one place where FOSS just isn't going to work. Let me ask you: are you willing to invest a lot of your time and effort into contributing to such a project? No, I didn't think so. Me neither.
Here's the problems with it:
1) It's not very fun. Volunteer coders don't want to work on something as deadly-boring as tax filing.
2) It's USA-only: lots of FOSS volunteers are outside the US. They're not going to spend their time on a tax tool they're not going to use.
3) It's constantly changing. The tax code changes every single year. Lots of FOSS projects become mature at some point, and only get maintenance or occasional feature additions. Most do not work according to a schedule. The IRS requires you to file your taxes on April 15, and they release the changes to the tax code a certain amount of time before that, so a project would only have a certain window of time to incorporate all these changes.
4) The risk to users is high, since there's no one really willing to guarantee this product. Of course, bugs in tax-prep software don't absolve taxpayers, but there's still a certain appearance that a big company standing behind a product makes it safer.