Why are so many Discourse members being mailed? What are they being mailed for, exactly? That’s the more relevant question.
On a typical Discourse install, the only “all users” email is the digest, and even that is specific to only users who have not been on the website in a week or more, but also haven’t been gone more than a year.
In general you only get emails when someone is speaking directly to you, and even then only when you haven’t touched the website with any web browser in the last 10 minutes.