The offensive players always know where they are going and almost always running forward. The defensive players are in reactive mode most of the time. Backpedaling, changing directions. It wears more than simply running a route.
But on the LOS, consider that on passing plays, the OL is standing up and attempting to form a wall, while the DL is pressing forward, attempting to muscle their way around the OL. Those guys are going to get worn down quicker than the OL, every time. And on running plays, they are still attempting to press up field while the OL is firing off the line trying to knock them down. In other words, the DL exerts and loses more energy with the way they have the play vs. the OL. And that is where the real differential comes in at - the line of scrimmage. You'll see it a little more with LBs, as they're getting knocked around attempting to blitz or getting winded chasing around RBs and WRs.
The nature of the movements benefits the offensive players. It's why you see the DL getting rotated in and out, and the OL almost never substituting.