There is a much greater depth of detail that I provide to my tax preparer than I'd be willing to share with the IRS. I tell my tax preparer everything and have perfect confidence that my taxes will be prepared correctly and in accordance with the tax code and that nothing which is not required to be disclosed will be disclosed. Much of what I provide to my preparer is irrelevant to my tax obligations and anything that's irrelevant I would prefer not be disclosed.
I'm mostly a W-2 schmo without any particularly complex business arrangements and with a spouse who employs themself in a consulting capacity, puts the statutory maximum into tax-deferred accounts every year, and we have minor kids who are required to file returns due to kiddie tax laws.
I can imagine many people would rather pay a high three or low four-figure per year bill for tax prep and representation rather than give the IRS full access to their financial lives.
I do support (and strongly so) the idea that the IRS could provide simple, default tax prep based on the information they receive. Where I break is what escalation path should exist; I think there must remain an effective and private escalation path for more complex scenarios. (In many ways, that makes it even easier to implement. Make the simple case automated-by-default. Punt all the complex cases to the current system.)