Just look at industrial controls. At the base you can say “air gap the system and it’s pretty secure.” Then you add on reliability, maintainability, optimization, etc. The people responsible for maintaining the control system are not IT, so they deploy a concept OT (operations technology). Now OT and IT have to play nice to deliver business solutions that make everyone feel warm and fuzzy about stability and uptime, but in reality the overhead and complication actually diminish returns usually.
The clash between the two often results in an endless cycle of trying to achieve security and efficiency, but since the two groups are beholden to different incentives they are regularly not aligned.
This often leads to reimplementing solutions regularly in an endless churn. I’ve worked a couple times on projects to replace solutions that have not been fully developed.
The issue is all answers are correct. A lot of the differences between the “standard” and whatever is out there come down to politics - whatever is being done somewhere isn’t necessarily there by design, it’s just there.
Usually, 80% of problems are solved by the big consolidation play. The 20% is the problem, because those are either complex technical problems or intractable political problems. Nobody can get a budget justified to spend more money on 20% of the environment, and the the people running the consolidation project push until they declare victory and get promoted.
I don't think it's because the people involved are completely insane; they see that IT projects in DoD are expensive, and the expense is seemingly due to government/DoD overhead not intrinsically associated to solving the software development problem. So if you could increase the amount of software you can deliver with the same overhead, it should be more efficient, right?
Of course, it's nearly impossible to make this work, but the real solution (significantly reduce the overhead so that it's again efficient to build smaller solutions and iterate improvements from there) is impossible to imagine in DoD.