Why do most people call people in the army heroes? When most of them haven't performed a specific heroic act.

Politics not withstanding, people who join the military are voluntarily agreeing to put themselves in harm's way and risk injury or death to protect the interests of the citizens of their country. At home we have the luxury of debating whether or not a war is just or necessary; on the field they have to trust that it IS, and if lives are sacrificed it will be for a good reason. That's pretty heroic.