The W-2 case with maybe a 1099 and standard deduction is pretty simple. And should probably be even simpler by autofilling a "Here's what we think you owe/are owed" return. But you get into complex returns pretty quickly for the reasons you describe. I don't even have that complex a return and what I got from my accountant was probably a 1/2 inch thick last year.

This isn't true. Lower income people are more likely to work more than one job simultaneously, or to do casual work or a string of jobs, or be 'contractors' than have a single W2 with accurate numbers. Only 20% of taxpayers who qualify for free direct filing use it - mostly because they don't even know it exists. Many Americans are challenged by even the process of completing the form.