I can't speak for the GP, but I think you're conflating "not all that complicated" with "so easy even master_kaos could do it in ten minutes".
From my own experience, a person with a small business, 5 rental properties, a house, a recent divorce, and two kids in college has a moderately complicated return. Extremely complicated (and far out of my league) would be the 500-page returns of the top 0.1%.
There are plenty of mistakes that tax software can't help you with. Common issues I have seen: incorrect filing of amended W-2s/1099s (IRS now thinks you have more unreported income), failure to retain source documents, failure to take any deductions for 1099s, failure to deduct mileage/vehicle expenses. From some larger tax firms that employ barely trained seasonal workers: blatant lies that will get flagged.
If you get a single W-2, don't buy or sell securities, have no other income, rent a small flat, and are absolutely sure you don't get any other deductions or credits, file an EZ on your own. For anything else, I would at least consult with a tax firm that is open year-round.