Why can't I edit existing share content after created, only the settings?

Why can’t I edit the underlying content of a share, only the share settings?

As the content is selected, encrypted, infused with the authentication DNA and stored on the chain, one cannot re-edit the underlying content without essentially re-creating the share.

Therefore, that is the recommended approach. If the content must be changed (not just the security settings), simply delete the original share and create a new one based on the modified content. 

As noted, shares expire after 30 days so they will be deleted and erased in any case, preventing possible unauthorized access in the future.