Why is network sharing off by default?

Because your network is yours. Network sharing is one setting for your whole account, off by default, and it asks you to confirm before it turns on. With it on, other itogai users who have you as a contact can see you as a bridger and ask you for an introduction, and they see only a name, employer, and role for the person you know, and the fact that you know them. Your scores, your communication metadata, and your messages are never shown, and switching it off takes effect at once.