This is a particular artifact of the government system process. These are contracted pieces of work that Company A would deliver, Company B would administer, and Company C would be contracted out for additional work. Further, all specifications were created ahead of time because changes would cost extra. (Anyone who has done government contracting can talk to the shenanigans involved with it - I have not lived in this world for a long time.)

That said, we still do ad-hoc versions of many of these. For example, a system/segment specification today is an OpenAPI document between microservices. Most larger SaaS companies have the equivalent of a Software Configuration Management plan - Who can change terraform or a GHA, what are the standards that they conform to (linter, peer review standards).

at one point or another in my career (gov contracting) I had to write or co-write or review every one of these. and without fail, within 6-12 months they would be stale/inaccurate/obsolete/… the truth is, even on projects where sufficient time is allocated to write these, there is never (literally) time allocated to keep them up-to-date