How often does a player run 40 yards during a football game? OL, DL, LB never. QB less than 5 times per season. RB almost never. WR & DB a few times per game.
Just as the top 5 team weight lifting records are typically achieved by walkons every year, how fast some dude covers 120 feet in a straight line is basically just as useless. Save KO returners, it's a worthless test leftover from the 1950's.
Football player achievement is directly defined by sudden bursts of speed, measured from a dead stop and performed within less than 1-3 feet of space in all directions. Only then, if a player has the required QUICKNESS to compete at a meaningful level at his respective position, can their continued foot speed have any sort of relevance at all for the next 39 yards.
Forty yard dash = Favorite pizza in most cases.