Here's an interesting factoid that most track guys know: you can only sprint at max. speed for around 1.5 seconds. The remainder of a 100 sprint is either acceleration or deceleration (the latter despite your best efforts - and truthfully, trying hard makes it worse - you need to stay smooth and relaxed). Sprinters train for years to maximize speed maintenance, through repetition and technique training. Football is all about rapid acceleration and change of direction. So there really is a difference.
I have no doubt Metcalf (and others, like John Ross or Tyreek Hill) could be a World-class sprinter. But as many on here have pointed out, he would need to walk away from football (and the millions of dollars) to focus exclusively on track. To possibly have career earnings of $10 million.