Note that there's a whole lot of countries that send you a prepared tax return that you adjust if necessary, and most people don't need to. There's no fundamental reason we couldn't do this here.
Indeed, California even did it for a couple years: https://www.taxpolicycenter.org/briefing-book/what-was-exper...
Yeah, it doesn't work for everyone. So they could offer it to you, and you have the choice to click "ok", or to click "add additional income and deduction". If your situation is so crazy that neither of those options work, you can then do it the classic/hard mode way with an accountant.
It's how it works in plenty of countries. The US is a bit unique with one of the more complex tax code in the world, but it would still work fine for a large portion of the population.
The feds have no idea how your deductions (and to a lesser extent, your income) have changed from the previous year. They don't have a full picture of your finances, but you do. In the abstract, with those constraints, the system of "tell us everything we need to know, pay us, and you get in trouble if you lie so egregiously that you get caught" works pretty well.