Why does it have to be 5 off. linemen when def can have 3?

by rule the defense can line up in any style they want anywhere as long as they don't cross the line of scrimage before the snap. the offense by rule must have 6 men on the line of scrimage on every snap so to give better protection to the qb it's just become standard to have a set 5 ol and switch out a wr or te on the line.if the te is on the line then the wr must be at least one step off the line.