What exactly is Steam?
Maybe we can suggest a Mac friendly alternative.
Steam is a app/service which enables gamers to browse and purchase games, download (& update, if necessary) them. IIRC you need to be logged into Steam to play the games too, hence it does require an active net connection.
The big game on it was Half Life 2, but now there are quite a lot.
I think Apple's gaming support has always been half-hearted. Until Apple has a large market share they're always going to struggle to attract many quality titles; and Apple isn't joining a fight it can't yet win.
OTOH, the way iTunes is going, it might not be long before it becomes a software download site for the Mac as well as the iPhone, and could be a great games distribution, anti-piracy and online gaming service.