The issue is because you need to log in to play games, you have to get your steam credentials authenticated. If your steam account can't be authenticated, then the client won't be able to tell what games you're allowed to play. For obvious reasons, you cannot authenticate your steam login locally without the internet.
Once you authenticate the steam client with Valve's servers, you can turn it offline and it'll save your credentials for over a month.
Aditionally, if you just don't log out / shut down the computer after turning Steam into offline mode, you can basically just keep it active forever.