I worked on tax software and have seen these kind of efforts. Noble, yes. Short lived, yes. It’s relatively easy to make a free 1040 file. The real problem comes from all. The. Edge. Cases. 1) do you support xyz state? 2) what if I lived in multiple states? 3) what if i… And on and on. And it’s not just one and done. Every year IRS and states are changing their shit so you have to go thru and make all kind of updates…
Around 2007-2008, as a "side project" I tried to build a web-based payroll software for a single state. Me and the developer I employed were able to get to 80% real quick, with ACH and everything. But as you say, the final 20% (edge cases) are nearly endless and ever-changing. Even a single state ended up being way too much for two developers.
I then went and found out how many folks one of our competitors (a bit more mass market than us) had writing payroll software.
The answer was it was a six person team just to keep up with the government changes every year. We never bothered any more with that idea.
Payroll is surprisingly hard.