Well, aside from practical problems (apparently this [http://www.rockpapershotgun.com/images/11/feb/scipt.JPG] is the script for Deus Ex 1. I'd imagine Ion Storm or Eidous wouldn't want to make 10 copies of it for judging back then, although I suppose there's always sending Word documents over the internet now), there's also games that I really doubt would have scripts in the first place. For example, Valve games. Aside from falling foul of the "must credit writer" rule (hell, they don't even credit Gabe Newell for anything), I'd imagine that if they had a script, it would mainly consist of disjointed passages with little context, especially for something like Left 4 Dead.