Complacency, complexity, corruption, and conservatism.

First, there's a strong status quo bias. The system works as-is, and the tax code is also set up in a way that's relatively simple for the median worker, so there's not a huge benefit to a change. Additionally, the IRS may not have enough data because data collection itself is decentralized. For example, married Americans typically file joint taxes, but the IRS has no way of telling who a taxpayer's spouse is until filing time.

Second, a lot of the complexity of US tax law happens on the deduction side, rather than the income side. The US has last-dollar deductions from taxes, so deductions are worth proportionally more for higher-income filers, but the IRS won't necessarily know about charitable deductions or mortgage interest or home office expenses.

Third is the aforementioned corruption -- tax-preparation firms consistently lobby against any sort of automatic return. Their arguments are relatively thin: they claim that automatic returns would have people not claiming tax credits they're eligible for and that developing automatic returns would be a waste of government money. In the meantime, they've collectively agreed to a "free file" program for moderate-income Americans -- only the program is barely advertised and full of "pushes" to paid tax offerings.

Lastly is the ideology of fiscal conservatism in the United States. Anti-tax politicians (usually Republicans) argue that automatic filing would make it easier for future governments to raise taxes, since people would not have to calculate themselves how much they're paying in federal tax. This is often coupled with farfetched offers to greatly simplify the tax code by (e.g.) replacing it with a flat income tax that collects much less revenue.