This is the result of decades of Republican lobbying, legislation and outright sabotage. The philosophy, openly advertised by Grover Norquist's Americans for Tax Reform, goes like this: nobody likes taxes. So if people find the filing process difficult and stressful every year, they will be constantly reminded of this fact. This will in turn make them more open to suggestions and propaganda along the lines of taxes being something that must be fought tooth and nail at every turn, because let's face it, who wants the tax code to become even more complex?
In contrast, if the process is streamlined every year, most people won't even pay attention to how much they pay in taxes - which isn't great if your ultimate goal is to keep government as small as possible.