Probably because it isn't worth doing. Let's say you really wanted to make your infantry tactics classified at the secret level. Getting any security clearance takes a long time. Are you going to make every new infantry recruit get a secret clearance before they begin training? If not, how do you plan to teach them how to do their job?

Now imagine you want to send military trainers to help build up a friendly nation's military. What are they allowed or not allowed to teach?

It also isn't that hard for other countries to use aerial or satellite surveillance to see what you're doing. Infantry tactics need to be practiced outside. How will you stop other countries from observing your tactics during training exercises?

(In the UK the equivalent manuals are classified, albeit at a low-level).

It's a bit like knowing the general rules of chess vs. having a specific gambit for use in a particular game. Some of the information in the manuals is common military knowledge, e.g. how convoys work, how to position forces making use of cover, etc. In effect the military laws-of-physics.

What usually remains classified are the detailed Tactics, Techniques and Procedures (TTPs) associated with given weapon systems or specialist force elements. Also, plans for specific operations are usually 'born secret' and tend to remain classified as that would give away thinking.

The same way I don’t need to keep the principle behind my house key or computer encryption a secret.

They’re quick to leak and near impossible to change once inevitably leaked.

I would assume the hardest part is sustaining the machine that provides the logistics anyway.