I'm curious what makes you think it's horribly coded. I actually work at Intuit on an app deeply integrated in TurboTax and although I haven't seen the code for TurboTax proper, my team's code is quite nice imho :)
For me, it was the deep integration with Internet Explorer browser elements and rendering engine. For several years, TurboTax just wouldn't run on my main desktop computer, always throwing some arcane error message about MSHTML, and the only info I could find from Intuit was something about upgrading Internet Explorer which was already done. I had to install and use TurboTax on either an underpowered laptop or my work machine.
It cleared up somewhere along the line, I think with IE8 circa 2010, but I'm always still not quite trusting of TurboTax's code quality.
