Sengoku and the marines have no clue about the void century. Only people that knew were Whitebeard (Roger's loose interpretation of it), Roger Clover and Nico Olivia now that they are dead. The only people capable of knowing would be the Gorousei, and Rayliegh ( he wants to tell it but only to people who are worthy Luffy is worthy, but he wants to no hisself ) , and Shanks ( he was a cabin boy and they like to listen on to things hmmmmm).With Whitebeard dead and the majority of the Roger pirates missing or dead, Oharan's wiped out. That means the only people who know are the Gorousei.
Take a look at the Gorousei's physical appearances if you look at them closely the fit descriptions of people like Mahatma Ghandi, Hallie Selassie, Mikhail Gorbacheuv,Odin, the blonde (I can't figure out yet). People who were said no about human enlightenment, and secrets of the world. Now all these men have large visible scars. These means they have either traveled to Raftel or engaged in battle sometime in their life. Now why would these men who have the power to call on an Admiral and buster calls for protection need to fight, The reason for this would be because they needed to get the information themselves about Raftel not trusting anyone else to go along with them to acquire it.
Dogs of the Military like Akainu are easily kept inline with the easy black and white description of justice and life. If somebody like Akainu who is willing to kill subordinates for desertion finds out his government is a sham then he would probably go on a rampage to cleanse the government of its sins.