It's just a bit too much grunt work to make something that lots of people really want to use. Tax codes change very year, there are 50 states in the US with different tax codes, it's a lot of work to review contributions (are you a tax lawyer familiar with all 50 states ...)
Lots of successful open source projects are started by one person, who spends a few years making something very useful for themselves, and sharing it, then slowly growing the contributor pool. But this kind of project is a lot more work than just doing your own taxes! And nobody wants to use it as-is 2 years later, it needs more maintenance work every year to be baseline useful, a lot more than it takes to just do your own taxes.
Moreover, experts like tax lawyers don't have any incentive to contribute; if the system turns out to be that simple and effective, they don't have much left to do.
They didn't feel the urge for needless responsibility, is what first comes to my mind. Many open source tools, like GNU grep that I just checked, contain text that points out that they take 0 responsibility for the correctness of the software. In case of a serious thing like taxes, this is even more important.
I've felt the urge to make it for a long time, but there are other things I was more passionate about. I did actually start looking into it this past tax cycle mostly out of anger/spite at Intuit, but I'm glad the govt is doing something now. I think it should be open source.
Source-available might be more practical in this case. Low-quality PRs and issue reports can be a massive drain on a project. A IRS tax-payer tool is about the most likely project I could think of to attract massive amounts of that.
I would imagine because of the self barter nature of open source. Open source only works when the group that can make the tools are also the group that uses them. So they are bartering with themselves. That's how they "pay" for it. But contributing back to the shared pool of resources
Taxpayers can't contribute back to the pool that software engineers use. The people who build it will not be "compensated" with other tools that they can the use for free. So they don't build them
>Open source only works when the group that can make the tools are also the group that uses them
Counterpoint: Blender is a wildly successful open-source 3D modeling tool. Yet the majority of Blender developers are not 3D artists, and the majority of Blender artists are not developers.
Maybe it should be open source but read-only. The only opposition to a government filing system that I think makes any sense is that we shouldn't necessarily trust the government to get the taxes right*. Making the tax software open source (or public) would mitigate that concern.
* - It's not something that I'm concerned about but I think some skepticism of the government is healthy if easily addressed.