Because of regulatory capture. In essence, TurboTax has been able to purchase legislators who then ensure that tax law is difficult for regular people. This creates a larger market for TurboTax. That's similar (albeit less awful, I admit) to how for-profit prisons are able to purchase legislators to ensure that they are able to fill their cells.

There are some countries that have managed to avoid having tax prep software companies purchasing their legislators. In those countries, the best way to file taxes is basically observing the overview and clicking "yes that's accurate" on a government-provided website.