Earlier quoted context omitted.
The US individual tax reporting process is an annual reminder to voters of what they pay in taxes, that cannot be easily ignored. I am sure that fiscally conservative politicians see this as an advantage to their cause, though they are unlikely to publicly say so.
Why wouldn't an IRS-prepared return do the same? You'd still have to look at the numbers and handle your payment/refund.
My guess is that relatively few people, of those who currently prepare their own returns, would invest as much time and effort into verifying an IRS-prepared return as they do in preparing one. I would also guess that a majority of swing voters prepare their own returns (I know, that's a fair amount of supposition on my part...)