Roughly 62% of Americans are homeowners[0]. Somewhere between 50-75% of Americans have children[1][2]. If you're in either situation, you're likely unable to file a Form 1040 EZ. While the full Form 1040 is only 2 pages long, the instructions for it are 106 pages long[3] - and when you complete your 1040 you declare, under penalty of perjury, that you filled it out accurately.
Those people may not technically need an accountant, but to do their taxes without assistance would be foolish.
Neither of those situations is particularly difficult to handle. My mother has all of those things and to this day she still files a traditional paper 1040 like she has for the past decades. And she's not college educated or especially bright. She just does it because an hour of her time is worth more than the nearly $200 all-in costs of doing a state and federal return with TT.
The IRS could go towards a model where they assume a standard deduction and verified dependents, and issue a refund based on that. Then let people file an amended return with itemized deductions if they wish.
The tax code in the US can be very complicated, but the for the vast majority of wager earners, it's pretty straight forward. The complications come on the business end of things.
You could use this phrase to describe a lot of advertisements and products. However, these ads work on many people and they are made to feel like if they need it. Considering the argument they make is that with TurboTax you could get more money back than without using it, I am not surprised many people buy this tool and use it.
Very few people actually need an accountant or even TurboTax to do their taxes.