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.
There's also "Free File Fillable Forms", which is part of the weird compromise that tax return prep companies like Intuit and H&R Block negotiated to prevent individuals being allowed to file directly with the IRS. It's actually operated by one of the companies in the "alliance", but it's set up like a digital version of the IRS forms (up to and including the fact that you need to make your own copies of everything because they delete all accounts/data every year).
Software dev thinking that tax code is determined by software lobbyists, is like medieval peasants thinking that thunder is determined by a god that is angry at them.
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.