because it's not practical to write a Google Maps (or whatever) replacement most of the time? It's a ton of work. Sure we can smooth over rough edges but actual

because it's not practical to write a Google Maps (or whatever) replacement most of the time? It's a ton of work. Sure we can smooth over rough edges but actually implementing lots of missing non-trivial things? Or, making things faster that rely on some 3rd party back end? Usually either not an option, or not an option without recreating the entire front end.