I can see why you might think that but I believe that's actually insurance against Microsoft going the Apple route and hamstringing Steam in the process. They needed a near first class platform that would never be used against them to exist and they needed the switching cost for end users to be near zero. By leveraging pre-existing FOSS projects they managed to avoid the vast majority of the development costs which would otherwise have been prohibitive.
I can see why you might think that but I believe that's actually insurance against Microsoft going the Apple route and hamstringing Steam in the process. They needed a near first class platform that would never be used against them to exist and they needed the switching cost for end users to be near zero. By leveraging pre-existing FOSS projects they managed to avoid the vast majority of the development costs which would otherwise have been prohibitive.
The best insurance against monopolistic behavior is to get there first.