An NFL game consists of a series of designed plays, and the 12 minutes is just the execution time. In between plays, there are formation changes and setup, so

An NFL game consists of a series of designed plays, and the 12 minutes is just the execution time. In between plays, there are formation changes and setup, so fans can guess at what the offense is planning and how the defense is reacting. (That happens in other sports too, but here there's a formal beginning and end to each play.) Then there are also breaks between quarters and after changes of possession, where genuinely nothing is going on.