Not only that, Valve has done a TON of work to outsource as much of the process of running Steam off to the users and developers. Self-publishing, a minimum of manual moderation, automated greenlight processes, automated ratings, database tags, controller configs...
Their entire business model is to make money with as little effort as possible. I've been saying for ages that people vastly underestimate how ruthlessly profitable their business is. We didn't have the numbers, but we roughly knew this is what was going on.