It’s a safety check to prevent data loss. Imagine a drive with a syncthing folder getting silently unmounted or getting unreadable somehow - if syncthing rescans data, it might think that the user has deleted all of his files which would then propagate to other devices - resulting in possible data loss. Therefore the .stfolder is always checked for presence, to ensure that the directory is in fact readable and deletes are actual deletes and not errors.
If the folder is not present, syncthing assumes the folder has experienced an error (drive unmounted or similar) and will therefore stop it.
That’s why syncthing will never attempt to re-create the folder on it’s own (at least not without user interaction), as that would break the purpose of it.