E.g. the State of Oregon contracted with Oracle to create a health insurance marketplace. $200 million dollars later there was nothing but lawsuits. https://en.wikipedia.org/wiki/Cover_Oregon
Let that sink in. Two hundred million dollars and they couldn't get a fucking website working. Two hundred million dollars!
Anyone who has played with TurboTax or H&R Block software knows that the software is at least pretty good. There are plenty of nuances in the tax laws, and the software at least attempts to cover those. Sure, a good tax accountant probably can do a lot better, but for 98% of people the software is good enough.
Does anyone seriously believe that the IRS would do better? Why waste taxpayer money to find out?