Technically it's a guild, not a union. It's good for things like providing health coverage and making sure writers get paid and get credit. From what I understand, it has not been great at ensuring sane working conditions.
Basically being a TV writer is like working in video games; there's another person right behind you who would love to have your job because of the perceived glamor so terrible conditions persist.
Ultimately what drives screenwriters is their need to create. For those of us who do creative writing, be it for screen, dead tree, or compiler, there's this internal need for the work to make itself manifest. We're just the conduit for the story. That's why people put up with the conditions in the writers room, it's all for the story.