O Line rotation?

[spiketex]

Excuse my ignorance, because I never played the game and grew up in a foreign country. But why do most teams rotate Defensive Linemen but not Offensive Linemen? The big guys who are all over 300 lbs may have more strength and energy to repel the Defense if they had some breathers.
I welcome your feedback why this doesn't occur...

 

[dablur]

The o line doesn't run near as much as the D line does. The D line chases the play almost every play.

 

[dlockz]

Like the other poster said it about keeping legs fresh. Defenses have become very specialized. Its more of a big man thing because Corners run much more but seldom do teams continully rotate their corners