The problem with pre-populated online forms is that if there are issues with the data that the tax authority receives from third parties, proving it wrong is difficult, and the burden of proof shifts to the taxpayer.
A few years ago, I resigned my job and I transferred my pension to what's called a Preservation Fund. In South Africa, you can get your pension paid out when you leave a job. Getting a pension paid out incurs tax, while transferring to a preservation fund is tax free.
However, the company that administered the pension fund screwed up, and issued a tax certificate that treated the pension transfer as a taxable payout, and submitted it to the South African Revenue Service (SARS). The pension administrators proved to be incompetent, and were incapable of fixing the issue.
After months of battling, I managed to get the issue sorted out, and I got a refund from the taxman, rather than the hefty payment that was initially assessed. Other colleagues who also left had less energy than me simply paid the tax caused by the pension company's administrative error. I'm sure there were hundreds of people around the country affected by the error, and most must have rolled over and paid.
I've read that one of the objections to having a pre-populated system in the US is that people are less likely to object if the forms are pre-filled. My experience would seem to confirm that view.