So why hired named writers? Marketing appeal? Do regular people even care who Stephen Gaghan is?
That depends. For Hollywood, there's Writer's Guild influencing a lot of things. In gaming, not so much - largely it's names that can generate a buzz or appeal to a certain audience, otherwise, it's a large amount of "no-names".
AAA games as such are not author-centric. There are some exceptions, but these are far and few between - just like game artists merely creating textures, so are the writers merely creating a "narrative". It's all part of a complete design, and depending on interference from the publishers, shareholders, finance, time frames, project leads and such, the writer role can diminish to essentially that of a monkey with a typewriter.