Because once someone posts or comments anonymously, they can only continue to post anonymously on the same thread. Then that choice stays until one actively switches back to comment from their profile. Some people do it because they might want to speak openly, perhaps putting personal details they don't want to share publicly, or because there's a lot of abuse on some of the threads. I don't understand why it bothers people so much.
Best to provide the most recent 6 months. If over a week old, I would add a screenshot with your current balance. If applying under workcation, you should highlight income receipts on the statements.