Valve are the only ones confident enough in their systems to do that. Valve’s mindset seems to be that trying to lock people in is a losing strategy, long term. Instead they are just making sure that their offerings are better than anything else available. If done right, it has all the advantages of locking people in, with none of the downsides. It also combines with the perceived openness, which gains you a lot of credit with the geek community.

Microsoft are too reliant on lock-in to risk opening it up.