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 n

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.