Because that's where it started from.
All passes, runs, and kicks are calculated from the line of scrimmage. It is the one arbitrary line that can be used as a universal point of reference without subjective decisions, and thus can be universally applied. The exception is the field goal; it is calculated from the location of the ball when kicked, except that if there is a penalty, it is still calculated from the line of scrimmage.
it is not legal to drop kick beyond the line of scrimmage. all you can do beyond the line of scrimmage is pitch the ball backwards.
there must be 7 men on the line of scrimmage, no more no less
Line of scrimmage.
on tv, the line of scrimmage is usually blue and the first down line is usually yellow.
A passing yard is the distance a football is thrown from the line of scrimmage to where it is caught by a receiver. It is calculated by measuring the distance between these two points in yards.
the line of scrimmage
The line of scrimmage.
On the offensive side, nobody has to be in a down position on the line of scrimmage, but there must be six men on the line. The defense does not have any regulations as to where they must line up on the line of scrimmage.
In the NFL, there is no such thing as pass interference (PI) on forward passes that do not go past the line scrimmage but I recently watched a college football game in which a PI was called on a forward that never reached the line of scrimmage even though I always believed that there is no such thing as PI if such pass was in the backfield (behind the line of scrimmage). So I am not clear on the rule concerning passes that do not cross the line of scrimmage, in college football.
The offense is allowed up to ten players on the line of scrimmage, need one to receive the ball from the center. But the offensive is required to have a minimum of seven players on the line of scrimmage. Defensively all eleven can play on the line of scrimmage.
10 yards from the spot of the foul. So if the hold occurred 5 yards behind the line of scrimmage, then the penalty would be a 15 yard penalty from the previous line of scrimmage. The down would be replayed.