@Edent probably because it's very easy to leave code in a non-compilable state in the process of typing in your changes. The other coders in the team won't be able to compile or prepare for deployment until the code at least satisfies a syntax parser.
The discipline of manually committing changes, (and pushing / pulling / etc) means it's easier to hook in code quality checks before code is deemed 'ready' for production.

@zymurgic @Edent Yeah that's the obvious reason why "everyone edits all at once" wouldn't work, but I wonder what conditions could be created to allow it to work. I'm working on a very "broad" Ruby On Rails app, meaning we have many many views, controllers, and even many models, coding different parts of a big sprawling site. MVC style "separation of concerns". I think a few people *could* edit different files in parallel, but even then, make a syntax error in the User model it all tumbles down!

@Edent @zymurgic Sure the shared editing tools exist. I had colleagues who tried using a plugin for Visual Studio code, but decided they preferred shared tmux sessions, however...
they were only using this for pair programming sessions, coding together on something related, while on a call, and then committing their changes to git as usual.