Actually, I believe TurboTax has successfully lobbied congress so that it's illegal for the IRS to do that.
https://www.propublica.org/article/inside-turbotax-20-year-f...
Discussed at https://news.ycombinator.com/item?id=34764952
Building the thing is going to cost a hell of a lot more than 15 million dollars, it's not a 3 man startup android app. It has to be reliable with high uptime, it has to cope with enormous loads on April 15, it has to be accurate and have no potential for data loss, and it has to be Made in America, no outsourcing for various reasons.
Many smaller nations have had boondoggle affairs with MUCH smaller systems.
McKinsey would have likely charged triple.