Very true, but there are still some games, that are more realistic, than others, so you can learn real tactics from them. And I enjoy them more, than the "cool" games, despite having no intentions of ever being close to a real battlefield.
- Operation Flashpoint, a shooter (later Arma) is an example, that is also actually used by some real armies as a training simulation (but in a modified version).
- Close combat series, is a tactical strategy game, where you command small units in WW2. (which can run out of ammunition, or simply run away or surrender, if they are under too much pressure)
- Total War series, for big old school battles, with Units in tight formations against each other.
And about realism and coolness and war: I actually think, all those glorifying Call of Duties etc. for example are really dangerous for the minds of the teens playing it, as they show mainly the glorious side. There are no civilians for example. You can just blow up allmost anything and the game rewards you for it. (mostly)
In those games, I never had a moment, like I had with Operation Flashpoint. In one add on, you fought as the red army against resistance. So people in civilian clothing, but some with guns that want to kill you. And this is what matters, since one bullet can kill you - so you shoot at them. At anything that moves. But after the battle you see, oh. Most of them were really harmless civilians. And you shot them, because you were scared, that they shoot you. There was no glory in that. But it left a deep impression with me and I started to understand, what collateral damage really means. And how it is impossible to avoid them in an urban war setting, like in Afghanistan or Iraq.
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?
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.
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.
If you have specific questions about how military units are organized in the modern era, I would really suggest downloading US army manuals on various topics from the era that you want more information about. For example, if you want to learn how US officers were trained to attack a city, download the manual about it, and it talks about what infrastructure to blow up first, how to handle communications, what formations to use, and so on and so forth. It's a lot more direct than reading a conventional history of a war. You can get answers to questions like "how did soldiers learn to move with stealth during the Vietnam war" by reading the manual more efficiently than you can from general histories.
I really recommend reading military material and not trying to glean things from games or fiction. The latter lean heavily on rule of cool versus rule of not dying and killing the other guy more effectively.