Yes it's a steel rack. It's one of the reasons I prefer it over the Surco. The surco design is good for most moderate load situations but if you have to carry an entire rear end (diff/axle housing and entire assembly ratcheted to my Garvin Rack) up top like I did on one of our trips out, I'm not sure how well the Surco rack would hold up. From what I saw of them, they're not built to support that kind of load off road.

Another factor that also comes to mind is simply metal fatigue which I'm told with all things being equal is far greater with aluminum than steel.

If these aren't a big factor, then I'm sure Yakima, Thule, and other rack makers would offer aluminum based racks instead.