Writing isn't just the words on the paper. It's that, of course, but it's also plot construction, integration of themes, narrative and gameplay, and character development.
I'm not sure what you remember of Psychonauts but it didn't have that little writing. Of course, it didn't have as much writing as Bioware's games but still had more than enough to make a critical appreciation of it. Just the intro sequence, for example, was better written than anything Bioware has ever done.
I don't dislike Bioware, by the way. BG2 is one of my favourite games ever. That doesn't change my opinion that Bioware's writing is mostly just functional, including in BG2.