Yglesias's answer:

[F]or the vast majority of the population, most of the pain of tax compliance could be eliminated by a few keystrokes at IRS headquarters. So why don’t we do it? Two reasons. One is lobbying by the tax preparation industry to discourage states and the feds from developing easier tax-paying systems, as California recently did. The second is lobbying by anti-tax conservatives. When the Golden State implemented its ReadyReturn system, it did so over the objections of Grover Norquist and his anti-tax pressure group Americans for Tax Reform, which fears that if taxes become less annoying voters might be less unhappy about paying them.