An actual argument I've heard from TurboTax shills is that tons of people underpay taxes now with the self report system, and having the government pre-fill forms for you to rubber stamp would effectively increase taxes on the working class.
About 20% of people (particularly low income people) have simple tax returns and file using 1040EZ. If the IRS computed that form, then what they send out would be ideal for the majority of taxpayers. So it would benefit those by eliminating the expense of tax preparation without actually costing them more in taxes.
Then there are those like myself who fall into an in-between area. I have more complex taxes, although not as complex as a lot of people, but would gladly pay more than is strictly necessary in order to not have to bother with figuring them out. In fact, I already do, in the form of paying a tax preparer (I fully consider the expense of tax preparation as a tax itself.)
Those whose taxes are more complex are very likely aware of that fact, and would continue to do their own tax preparation.
Also, if the IRS provided a prefilled 1040EZ, that would make computing your own taxes easier because you could use the figures supplied as a reference.
2017 Form 1040EZ goes up to box no. 14
2017 Form 1040 goes up to box no. 79
2018 Form 1040 has no boxes for $ amounts!
2018 Form 1040 Schedules 1-3 is where they all went, if you needed them.
2019 Form 1040 added boxes back up to 11b
2019 Form 1040 Schedules 1-3 also exist
Except many people will try TurboTax or a competing solution anyway, just in case they can get a little bit of money back.
Tax evasion and overstating deductions are common partly because the IRS doesn't have access to the information necessary to verify everything -- that's the same information the IRS would need in order to pre-fill tax forms accurately for the 1/3 of taxpayers who itemize their deductions. 1/3 of American taxpayers a pretty big market for TurboTax and similar services to thrive in.