When the 1040EZ was a thing, only 16% of filers used it. Those would be the candidates who could safely have the IRS do their return. With anyone else, there's all kinds of information the IRS has no clue about.
But the IRS still knows about them.
Also they could put a website where you could spend five minutes entering the most common information they don't already know, and then spit out your bill.
It's not that hard. Most filers situations aren't that complicated.
That's basically what they're doing.