A few years back I got audited, and it was called an "electronic audit" by the IRS. Which basically meant that they had their computer systems scour a bunch of the bank records and discovered that I had forgotten to claim a 1099 for some contract work I had. OK, so they sent me a computer print out and they had cross checked everything on my return electronically, my property tax deductions, my stock dividends so on…
It's impossible to automate the tax filing process unless you simplify the tax code. And I've filed taxes in countries with simple tax codes where the government pre-fills out the forms. Guess what? You still need to go and double check everything because the information comes from your employer. I've found mistakes before. So I pretty much "do my taxes" myself to make sure what the govt sends isn't wrong.
The gain is that we can make "doing your taxes" a non-issue for most Americans. Sure some people will still need accountants for complicated situations but in one swoop we can eliminate hundreds of millions of hours of useless work every year for basically no downside -- it's pure gravy.