WHY TAXES IN THE US ARE COMPLEX
The US governing mindset accepts that people follow financial incentives and disincentives (capitalism.) Thus the government can encourage or reduce certain activities via tax code treatment.
Some examples of activities that the government encourages via the tax code are: marriage, having children, taking care of aged relatives, donating to charity, driving a low emission vehicle, home ownership, getting an education, and investing for the long term. And these are just the simple ones that most people encounter, there are many others.
Further complexities stem from attempts to make the code fair. For example, if your work requires you to spend money on things like uniforms and certifications, the code has provisions to exclude that from taxable income. Likewise, you don't get double-taxed on foreign income you had already paid taxed on abroad.
Finally, the US is a diverse country with diverse values. Each state and municipality has a different philosophy on taxation and what the tax code encourages and discourages. People to some extent migrate towards tax regimes that align with their values, but of course this is another source of complexity.
The bottom line is that the US tax code is complex because it tries to do more and has to account for more variability than perhaps other tax codes out there.
ARE TAXES HARD?
The answer is: it depends. For the simple person who draws a salary and that's all, they can fill out their taxes in a few minutes, for free online - or even by hand in a form with no problem. I did this when I was a college student and early in my career. I often hear foreigners brag how their tax form is 3 lines - that's great. For many in the US, it's not much more than that. But, as you get into more advanced scenarios and want to take advantage of government incentives (or the government wants to make sure you pay your fair share in some otherwise tricky cases) you have to put in more work.
DOES TURBO TAX SUCK?
It's easy to find faults with most software, but the bottom line at least in my experience is it has scaled very well as my tax situation grew in complexity in different ways. The fact that one can rely heavily on this program for a workflow that has implications of tens of thousands of dollars every year is pretty meaningful.
CAN IT BE REPLACED?
Good luck. For very simple cases I mentioned above, sure - and there are already lots of cheap and easy ways to file in those cases, including trivially on paper. But once people have complex situations and many factors intersecting, your software needs to account for all of those factors well. That takes tremendous work year after year - to understand, encode, and test all of these things. In my opinion that's exactly the kind of project that benefits from commercial investment and wouldn't do well as open source.