Having worked on financial code that is brittle and subject to far less minutiae than the multitude of IRS code, I can be very sympathetic on the complexity involved. It is hard to handle all of the complexity in a generic maintainable manner. I once ran into a 56-page printed function to calculate the margin required for a portfolio at a prime broker. Was it poorly written? Yes. Was it also representative of the complexity of the problem? Yes. Not an excuse for poor design, though.

I imagine processing the data, deciding whom to audit, and tracking the process is more complicated. Heck; they're probably using ML for audit red flags