It wants to, but there's a vocal lobby against it -- Intuit, H&R Block, and 1.7 million accountants that would like to continue being paid to fill out the forms.
Do you happen to have a citation? I would like to know who in the government is pushing for this, and how they are proposing it be done (I do not want to see a Windows binary or some Flash applet; I am thinking more along the lines of Prolog or a similar declarative language).