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

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?