Today colleges start looking at potential recruits as early as junior HS. Young guy are thinking about a serious football future early on.
Christian McCaffrey was a good HS sprinter, as was Robert Woods, Sammy Watkins, Tavon Austin, Ezekial Elliott, Todd Gurley, none of them ran track at the NCAA level. Bo Jackson, Deion Sanders, Herschel Walker, Curtis ey, Darrel Green ran track in college and played football. And as a group they were faster than those mentioned above.
We aren't seeing faster RB's/WR's today. There are no Cliff Branch, Mel Gray, Isacc Curtis, Bob Hayes, Jamaal Charles, Chris Johnson, Curtis ey.