Hiring a writer can actually be pretty expensive. A low-end (but successful) ghostwriter can charge $5k to $10k for a single short work. Ghostwriters also, typically, need the “author” hiring them to provide a full outline of the story. Hiring writers is not “I have an idea” and the ghostwriter comes up with a story based on that kernel. It’s work for both entities, and if Universal isn’t going to get any actual ROI on it (see the in-park games and story-dumps), then it’s not worth the effort for either side.
EDIT - Source? My wife is a ghostwriter.