Why is it expected that your relationship partner/SO know your finances (in this case, a woman) if you have money??? Would you ever tell them?

Just saw a post like that and I was wondering why so many people are expecting him to tell her and to tell her as soon as possible as well as you can tell her any time but you MUST tell her.

Why is this the case? If it was you would you ever tell her? And if you don't and she calls you out on it, then how would you handle it or what would you say to her?