Understand that only personally people do things by choice. Collectively at any number, small or large, people do things not by choice but empirically, by something else which has no name yet. Let's call it, for now, empichoice (I just made it up by merging "empirical choice"). Some English expressions come close to the idea of empichoice. Status quo is close, but it is more static, while empichoice would include collective decisions. Collective decisions would be an adequate term, but it would imply that people collectively made the decision while in most cases; they were manipulated into making a decision (empichoice) which goes against their interest; so I will use the word empichoice for collective decisions implying that these are not real choices of the collective but most often - irrational collective decisions achieved by manipulation of public opinion. "Public opinion" is the term which is usually used by politicians and many humans understand that public opinion is most frequently a subject of manipulation. So empidecisions and empichoices should be used to describe human affairs instead of "collective decisions" and "collective choices" to indicate that in most cases the decisions and choices are made in conditions of very strong manipulation by negative forces, which are primarily MIC.
So the best illustration of why MIC evolved to become secret lies again in the fall of the Soviet Union. The whole history of communism was very educational. Many ideas behind communism and socialism are actually very good and are in perfect alignment with what is needed, what you and positive humans want to accomplish. The ideas of communism are of global brotherhood (siblinghood), a good of humanity and so on. Not only communist counties have these ideas. In all other countries, these ideas are present. The problem with the ideas of brotherhood and democracy in human society again lies in the fact that we are not telepathic or psychic and therefore are prone to deception. Every time and in every place humans attempted building a society based on principles of the common good, these attempts failed in two ways: these societies were taken over by military neighbors or hijacked by MIC. (Very indicative is the story of Stalin hijacking the Soviet system. If you may do your research on the topic, "Stalin: A Biography" by Robert Service.) So again, not by choice, but in an empirical manner, by the middle of the 20th century, the humanity evolved into a competition of two hijacked systems, called Communists and "the West" (the latter also called capitalist, free world, NATO, and other names). Both systems were hijacked by MIC but functioned in somewhat different ways. Although both systems positioned each other as radically different, there were a large number of similarities and despite the artificial isolation of the two societies, they somehow exchanged innovations positive and negative and developed very much in parallel sharing many traits. There were substantial differences in economical design in two systems, personal rights, rights of corporations, etc. Both systems were proclaiming the ideas of democracy and their governments, as always, pretended to be elected by citizens and to represent their interests. As always, in both systems, propaganda was strong for patriotism and against foreigners.
By 1953, the time of Stalin's death, an interesting thing happened. In part, due to economic reasons, in part, due to national differences, in Russia (Soviet Union at the time), in part, due to the active militarization of the government, the MIC involvement in government became open, while in America, the role of MIC although strong, was less pronounced. 1968 was the year of the most obvious differences. The hippie culture and peaceniks were active in many countries, including America, Czechoslovakia and even Russia, but in America, they achieved several victories. So by 1988, Russia became an example of a country openly run by MIC and America was an example of a country secretly run by MIC. Clearly, the citizens of Russia were more apathetic and less motivated to work than Americans and the Soviet Union failed to a large extent due to the social apathy of people who lost the belief in their government. There were other components of the failure, but the difference in motivation was obvious. The Western system, in general, utilizes an illusion of people working for themselves. The leading ideology is that humans are by nature selfish and therefore the private entrepreneurial spirit is what makes the capitalist system strong. In fact, most people in all systems work for similar large corporations controlled by MIC, but in the West, the illusion of serving self and being run by a good, democratically elected government is more pronounced.
Therefore the answer to the question: "Why MIC evolved to become secret?" is that in humans, the illusion of working for self is more energizing and motivating than open exploitation by MIC. In a small part, this working for self and human liberties are true in the Western world, but in large part, the economy, the resources, control, public opinion and public decisions are secretly hijacked by MIC; and this secret is unknown to the large part of the population. The extent of the hijacking is known only to the few people who focus on this research. David Wilcock, Alex Jones and David Icke are public figures knowledgeable and openly speaking on the subject.
MIC = military-industrial complex