Why don't Supreme Court nominees relax into a more revealing position?

In the WaPo, Ruth Marcus expresses her frustration with the way John Roberts and Samuel Alito talked about the role of the judge at their confirmation hearings. They presented themselves as humble, neutral interpreters of legal texts. She appeals to Judge Richard Posner for more insight:

As much as a court "is supposed to be tethered to authoritative texts," Posner writes, the Supreme Court often finds itself facing issues to which "the constitutional text and history, and the pronouncements in past opinions, do not speak clearly." It is in that "broad open area where the conventional legal materials of decision run out, and the Justices, deprived of those crutches, have to make a discretionary call."

Such cases, as Posner notes, inevitably bring into play competing conceptions of social good, without solutions that can be derived with certainty: the desire to ensure public safety vs. the need to protect those accused of crimes; the rights of the fetus vs. a woman's autonomy; the importance of colorblindness vs. a recognition of the legacy of discrimination; religion as a positive force in public life vs. the risk of marginalizing the minority. On a more elevated but even more important plane, different judges bring to the bench different attitudes about presidential power, federalism and constitutional interpretation.

What has been so disappointing about the nominees' testimony is their unwillingness to engage in this discussion in an honest, meaningful way. What has been so maddening about the questioning is the senators' inability to penetrate their platitudes or robotic restatements of the law. Because thinking hard isn't enough....

Can we picture a new, improved Senator who could penetrate the facade? Much as the Senators exasperate me, I think not. The neutral-expositor-of-the-law pose is so effective and appealing that it is highly unlikely that a nominee -- conservative or liberal -- will relax into a more revealing position. Posner's insights are important, but they are mostly accounted for at the point when the President makes his selection. He must pick someone whose instincts and judgments deserve our trust. Beyond that, the Senators may find things out about the leanings of the nominee, but, unless the President has blundered, they are unlikely to uncover something serious enough to overcome the sense that a highly qualified nominee warrants confirmation.