Upon @Xantec's suggestion I looked for the original script.   On [st-minutiae][1], we have a script labelled FINAL DRAFT dated 11/14/89 which actually contains this line instead!

> **Q**:  Humans are such commonplace creatures.  They roam the galaxy looking for something, **and they don't even know what it is**.
<sup>(emphasis added)</sup>

So this at least suggests that decisions where made by some combination of the actor and the director around shooting time to streamline the delivery.

Commentary:   The actual delivery is not only less of a mouthful, but it does sort of disambiguate the potential interpretation of the script line as "They roam the galaxy looking for something and when they do find it they don't even know what they've found" as opposed to "They roam the galaxy and they don't even know what they seek."   It's clear the intent is the second, which the actual delivery does clear up.

  [1]: http://www.st-minutiae.com/resources/scripts/161.txt