This is something I don't understand besides steam windows uses free programs like paint. If it's there you mysaswell use it/ I agree on that if steam gives it away they should use it.
Basically given that video games are increasing complex, and that players expect a certain 'baseline', it's basically common now for many game companies to use middleware for specific things. The idea is to trade money, for a savings in time. It's a process of determining whether the time/effort/money required to build a system from scratch is more expensive than just buying middleware.
* You could make your own physics engine, or you could just buy Havok middleware
* You could make your own game engine, or you could use Unity for low end and cost, or higher end ones like Gamebryo or Unreal Engine which can cost $1 MILLION to license
* You could create code for a scalable 3d UI design menu, or use Autodesk's Scaleform (you'll see this in literally EVERY AAA game)
* You could make your own matchmaking infrastructure and master servers and networking protocols with NAT punching for ease of use, or use Gamespy or Steamworks
* You can try to make your own achievment API with a centralized profile management. Or you can just use Steamworks.
* You can try to package and distribute patches to your users via you website, torrents, or build out your own custom client and infrastructure. Or you can just use Steamworks.
Middleware is so ubiquitous because it's cost effective for all the 'boring nuts and bolts' stuff. Remember that resouces/money/time is a finite thing for game development. So you have to choose where to spend time/energy/money and if the payoff for that is better bulding it yourself, or going with middleware. Steamworks just makes the money part of that equation really attractive because it's free.