With a novel you can create huge worlds on the page, go to outer space or run riot within the French Revolution. Writing about a child s birthday party is no different to writing about an earthquake. It all costs the same on paper. The whole point of a script is that it will be made into a film. Expensive events need to be justified.
Imagination is encouraged, but so is the reality of writing to fit within a budget.
The pages of a novel are the end result, but the script is a starting point. In mainstream Hollywood the process of rewriting a script can take collaboration to extremes. You have writers who specialise in writing action scenes, and they are often brought in to make chase sequences more exciting. Other writers are known for their flair with comic dialogue, and so are brought in to add sparkle to the lines. Then when the film is cast, big name actors often have their own writers who come in and rewrite dialogue to fit their speech rhythms, or to beef up their parts.