[KineticLensman]

The US often operates as part of a coalition and it is desirable for coalition partners to be able to understand how to cooperate with US army units. If you encrypt docs that you want to share with partners, then you have to share your encryption mechanism, which may be much more problematic.

Also, these sorts of docs contain general procedures, not the actual plans of a specific operation.

[aerostable_slug]

Some of these docs are literally how to frame a house. Or set up the plumbing for a communal latrine. Much of it is not sensitive.

[KineticLensman]

Yes. Even things like the average speed of an advancing column of tanks, or the largest vehicle that can cross a particular bridge, are going to be fairly obvious to any army that has its own armoured vehicles.

[aerostable_slug]

Fun .gov overclassification anecdote: I worked for a startup that had a contract with .mil and, long story short, we needed a snippet of code that described soil slippage under load, as it remains the gold standard in the simulation of various things including armored vehicles.

IIRC, it had been originally written in the 1950s by the Army Corps of Engineers in FORTRAN (then C, then we used it with Modelica). It was — and is — still export-controlled. I had to get permission to send it to our UK (yes, UK) subsidiary for fear that said deeply dark secrets might reach the eyes of our enemies...