The way the U.S. tax system is setup is impossibly stupid. The government already should have on record all sources of possible income you could be making since pretty much all places you could be making money are required to send it in anyways. So it's conceivably possible for the IRS to pre-fill your returns for you and have you just double check and add anything in that they missed -- but it does get complicated very easy outside of the trivial "here in my country they just take x% of my income". I'm going to guess that almost all of those comments are made by people who's earnings come almost entirely from salary/pay, but have no idea that lots of people make lots of money in ways that are not salary/pay. In some cases it's possible to make most of your earnings in ways that external to salary/pay and in some cases figuring out what that monetary amount is isn't totally trivial.

For example, this is all over the news right now, but suppose your company lets you drive a company car? If you only use it to drive from home to work or other business related meetings, you probably don't need to consider it "income". But what if you can use it for all manner of personal activities as well? Maybe then it needs to be considered for your taxes. So what's the taxable amount of the car? The MSRP x the percent you drive it to non-work activities which of course you keep dutifully and precisely logged? Why the MSRP? Most people try to negotiate car prices under the MSRP. Why not the Blue Book value? In the end, what does the government use to consider the car as income in order to tax you for it?

In other cases, property ownership, and the layered governmental system in the U.S. also complicated things. For example, you might get Federal deductions for taxes paid to local governments on property you own. But what about locales that don't tax property? More importantly, how does the Federal government know you own the property at all? What if you own it, but then "loan" it out to somebody who uses it exclusively? You're the title holder, but should they count it as income a la the previous example and now two people are paying taxes on one piece of property?

These kinds of scenarios are incredibly common and affect people in all socio-economic levels. These kinds of things extend to include retirement holdings, investments, cash transactions, different kinds of corporate bodies that can affect individuals, foreign business/income/investments/etc. and so on.

What Intuit did is take advantage of this messy system, and provide some level of organization on it so that people would stop seeing and paying for CPAs to do their taxes and could do it themselves. They then fed on this like a cancer and the tax system started to reflect and assume Intuit was a public service. Part of this was the fractured and uncoordinated nature of the Federal IRS, State Tax Departments, and local county and city level tax systems that don't cooperate at all even though tax laws that affect one may affect the others.

In effect, the IRS can't put out a complete tax system because the IRS can't process your local tax stuff - the locales need to. So the IRS has relied on external non-government organizations to do it for them.