Why OneNote just provide password protect section, not notebook?

Question:

I have found how to use password to protect section in OneNote, my question is how to password protect my notebook?

 

Reason:

Protecting section with a password is available in the desktop OneNote.
But OneNote doesn’t provide use password to protect notebook or section group, why?
It is because an OneNote notebook or section group is a Windows folder, and Windows folder cannot encrypt in general. An OneNote section is a .one file, so it can encrypt easily.