I live in New Zealand. I don't have to file taxes if my income is a standard wage my tax payments are automatic. My employer deducts them from my wage and they are sent to the IRD (Our government tax dept). My student loan and super payments are also automatic. If there is anything wrong then at the end of the tax year I can file to correct things. I also get donations rebates, this is a one page form that lists all…
The United States has a complicated tax code because it attempts to make things more "fair" for some people. Some of these are things people like, such as deductions for people with children, paying mortage, etc. Some are more dubious.
Your bank already has records of things like mortgages which they [can] report to the IRS. You can figure out two-earner tax rates with a program or chart. People already report their children in the W2's. The IRS already has the information on file and I imagine they use it to filter out folks that might need audited.
Even with tax credits, a complicated tax system, and all that, there isn't a reason why this can't just be done upfront instead of after the fact - at least for most folks.