But that's an operating system, not an application. You've got to give credit to the IRS spending more on one application than Microsoft spent on their OS.
OSes are not particularly complicated. I could imagine that tax laws require a lot more code (and tests) than dragging some windows around and scheduling processes. (Neither are easy, of course.)
That "one application" was software+hardware tasked with managing millions of hand-written paper forms. The electronic version of tax filing was far cheaper, and launched successfully.
