They should open source this. I would actually contribute to making this usable.
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
* - It's not something that I'm concerned about but I think some skepticism of the government is healthy if easily addressed.
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.
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.