google docs has a very nice version control
replies(3):
1. Lawyers don't want everyone holding the pen at the same time. Specialists want to make their changes and have them reviewed in isolation from other's changes. Think of a redline like a pull request. You don't want other people's changes in your work when it's getting reviewed.
2. Google Docs breaks down when even a single person drafts outside of it. See the section of my essay about why lawyers will never stop using Word. Even if you managed to get an entire firm using Google Docs, it still wouldn't work because they need to exchange drafts with external parties.