There would be no questioning whether this or that part of the tax code was interpreted correctly because, being the IRS, the calculations would be correct ipso facto.
The IRS isn't the final arbiter of what's correct though.
So if you disagree with the IRS's interpretation, you'd have to file a lawsuit, instead of them coming to you to try to collect the taxes they think you owe.

As it is now, a lot of people can e-file for free using somebody's software. And IMO that's a move by the software makers to make it less appealing to people for the IRS to do exactly what you propose. (Intuit, 2nd Story, etc. can lobby congressmen saying "you don't want the IRS to do that - we already provide this for the majority of your constituents, so it'd be an extra cost for the IRS thats currently being borne by private business"