Eventually, privacy was eroded away year after year to the point where the government now knows an extreme amount about each citizen.
I guess I'd refrain the question - instead of asking why do we need to do taxes when the government knows everything about us that they could do it for us, should we really have that little privacy? Maybe instead of changing the way we file taxes, we change the way the government is intimately entangled with our lives?
Of course, those ideas, liberty and privacy, small government, etc., isn't really fashionable with the latest generations and most people would gladly give up their freedom and privacy to save having to fill out some paperwork.