Why use the OneNote that comes with Windows 10 to open the attachments in its page that are read-only and cannot be edited?
Because the OneNote that comes with Windows 10 belongs to UWP software. This type of software can only run in sandboxes and cannot communicate directly with external programs, especially .exe programs.
This is destined that OneNote for Windows 10 cannot integrate with Windows normal programs like OneNote 2016.
So attachments in the OneNote UWP can only be viewed and cannot be edited.
And OneNote 2016, allows you to edit attachments and save them back to the original page location.