Because some douchebag might come along and manipulate them for political gain, misprepresenting what actually happened in court.  Like this ad from the RNC:

 As it turns out, the RNC's new ad is about as honest as the RNC's attacks on the health care law itself.

A Republican Party web-based advertisement uses altered audio from U.S. Supreme Court oral arguments to attack President Barack Obama's health-care law. […]

A review of a transcript and recordings of those moments shows that Verrilli took a sip of water just once, paused for a much briefer period, and completed his thought, rather than stuttering and trailing off as heard in the doctored version…. Recordings of the court proceedings reviewed by Bloomberg News reveal that the audio has been edited. While Verrilli paused once to drink water during the opening moments of his presentation, he stopped talking for only a few seconds before continuing with his argument. In the RNC ad, he pauses for about 20 seconds, seeming to lose his train of thought.

Emphasis mine.

Tom Goldstein, a Supreme Court scholar and an influential attorney who has argued 24 cases before the high court, called the RNC's ad "the single most classless and misleading thing I've ever seen related to the Court." He added, "It is as if the RNC decided to take an incredibly serious and successful argument that has the chance to produce a pathbreaking legal victory for a conservative interpretation of the Constitution, drag it through the mud, and vomit on it."