Think of the defensive positions and their job.
Defensive line has to try to overtake the offensive line by pushing and running around them. While the offensive line is just trying to hold their position.
Linebackers have to cover a lot of ground and attack runners, sometimes cover, and sometimes spy QBs.
D backs are and always on edge trying to cover because they never know when a wr is going to make a break.
Just overall, compared to the offensive equivalency positions, the defense has to work harder to have success. And it can be very tiring to be on the field doing that over and over.
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