The job is in fact not as stressful as it sounds though for most composers. Composers DON'T orchestrate the music. And often composers just hand an orchestrator a lead sheet with chords and a melody, and that's it. Some composers barely sketch ideas. The orchestrator does the rest. After that, a transcriber comes in and puts it to sheet music.
However, it's changed a bit with some composers that utilize DAWs. It adds more more work in the process.
Danny Elfman doesn't orchestrate his own music. In fact his orchestrator probably leads to the sound that he's achieved with some exceptions. He might learned to do it himself later.
Also, there are ghost writers that are hired as well. These are composers that write sections of score for the movie, but are never mentioned in the credits, so the composer takes the credit. The trade off is the ghost writer gets paid a hefty sum, regardless of the success or release of the project.