The more the offense plays, the more successful they are. When on a positive roll, you don't notice the tired as much. Physically you expend energy. Mentally, each play enhances your belief and positive outlook on winning the game.
BUT, going back to the Mason days that was one of our problems. In close, hard fought games, the offensive line got tired late. The lack of depth caused the run game to lose effectiveness at the worst time. They got tired and lost effectiveness (it happened with every coach, but Mason's running attacks were so successful it happened more under him).
The converse, the defense when they're on the field too long, is because they're getting beat and out schemed. That takes more out of you because the wearing down is both physical and mental.