Because the options are saved as options and the fields on the page are saved in post meta for that page.
If you want the options to be eitiable on both the page and the options page then you can set the “post_id” argument when creating the options page to the post ID of the page. This will cause all options saved on that options page to be saved to the post_meta for that post ID instead of in options. Changing the post ID for options page after the options are save will cause ACF to ignore that previously saved values.
Usually, when you create an options page it is because you want to edit the values separability from a page.
A field groups field values are unique to the place where they are edited… a page, a post, a user, an options page.
