Word processors typically default to 1" margins. Before when most documents were eventually going to printed this made sense, but increasingly documents are only consumed digitally. So, why devote valuable screen real estate to these margins? Text editors for software developers don't do this.
Layouts for documents that are meant to be consumed digitally and in print should be different; they're different media. While zero margin may be too stark for most people, consider smaller margin defaults for documents that are unlikely to ever be printed. Put differently, why leave space for page breaks when a document is being read digitally?
The concept of a page of digital text is starting to fade in favor of measuring text in terms of estimated read time for blog posts and percent completion for digital books. It's time for defaults to catch up.