Earlier quoted context omitted.
The people who have complicated taxes also usually have complex income sources and generally higher incomes. If you just get your wage the tax forms are relatively straightforward. Being international complicates things but you'll notice people are talking about the headaches associated with running their businesses, capital gains, etc.
Yeah, while you could argue that the IRS should do pre-filled electronic returns, the reality is that if you have a W2 and a couple 1099s, doing your taxes is pretty straightfoward. As you say, it's more complex income sources and potentially deductions that lead to big accountant bills.
And you mention 1099s, i.e. self employed. Well, if you run a sole proprietorship as an expat, you still have to file US self-employment tax forms, and there are also QUARTERLY filing requirements with penalties for non-filing.
Or you need to get the paper certifying that your country has a totalization agreement with the US (not all do).
And since 1099 isn't a thing in EU, you have to file the US 'small business' tax form, which is actually 3-5 forms.
so as an expat, even what should be very straight forward turns into a massive mess.