Yes, and if a software engineer writes a piece of software on their own time and it becomes a hit and/or gets purchased by a large company, they can make a lot

Yes, and if a software engineer writes a piece of software on their own time and it becomes a hit and/or gets purchased by a large company, they can make a lot of money too. But if a writer is hired as a salary writer for a show, they're very likely to just get a salary. Same for a programmer. Big name writers can negotiate a better deal for themselves and are a different story.