Because changes aren’t written to disk typically (unless you do a save, or save as). That means the “undos” are stored in memory only. When the application clos

Because changes aren’t written to disk typically (unless you do a save, or save as). That means the “undos” are stored in memory only. When the application closes everything it had in memory goes with it, only things written to disk (or perhaps a database, depending on the app) will persist between program restarts.