Because its closely based on real events that arent hero running into gun fire all 5min but even during an invasion a whole lot of downtime, prep time, time to make mistakes, to interact...etc.
The show is more about how the people interact with each other , how they approach their targetsm deal with the stress and the somtimes confusion regarding chain of command inside the marines but also interacting with other branches of military. Its not supposed to be a battlepic. If you arent interested in the characters they portray ok then the show isnt for you buts thats were its focused on and I have to say it does this aspect a lot better than BOB or pacific with their very hero like characters. But these shows have other strenghts. You kinda missed completely the point of the show.
Its like complaining about The Hurt Locker that there arent any tank battles.