For what it's worth a large amount of the DOE work (where they have the super computers) is open source. You got ORNL (with Summit and soon Frontier)[0], ANL[1] (soon to have Aurora), and LLNL[2]. I think what needs to happen is that things could be better organized, for example ORNL has [3] which still open sourced but not grouped under the ORNL GH. Also if we got to code.gov and search "ORNL" and "C++" we only see DCA++ which isn't on the GH but here[4].
I think as long as code isn't sensitive to national security (LLNL...) it should be open source. But I think the big problem is that organization and discovery is very difficult. code.gov is an attempt to solve this, but it doesn't do it well.
[1] https://github.com/argonne-national-laboratory