Why is incognito mode so difficult for browsers to implement? If the browser already comes with support for profiles, then isn't switching to incognito mode the

Why is incognito mode so difficult for browsers to implement? If the browser already comes with support for profiles, then isn't switching to incognito mode the same as running from the empty profile? In particular, why do particular APIs need to be shimmed or disabled? In my empty-profile based proto-proposal, even if a website writes to disk, wouldn't closing the session cause any data written to be rolled back?