They have a staff of 150 to publish 20 articles each month. That seems quite top-heavy. Remember that authors and reviewers of the articles are not paid.
https://news.ycombinator.com/item?id=16339193
Nature drops about 95% of submitted papers somewhere in the pipeline, so part of the costs applies to 20x more papers than you see in the end.
If an editor can survive with these fees even when the paper will be publicly available, how can Nature need $30,000 for a paper people will pay to read?
Ordinary scientific journals don't have reviewers in paid staff. An editor sends an article (without reading it) to 2-4 non-paid reviewers, and makes the publish / don't publish decision based on the reviews. Also the editor is not paid, either.