Because acceleration isn't a thing. Running routes, making cuts, and otherwise changing directions (necessitating some manner of deceleration and re-acceleration) doesn't happen....and some guys aren't better or sharper at doing those things, allowing them to stay with a guy who ran faster in his underwear at the combine. Pass routes don't incorporate deliberate changes of speed to accomodate a release or get a defender to flip their hips, change their leverage, sell a route then pop a double-move, etc. Pursuit angles aren't a thing either.
Oh, and splits don't exist either...if a guy ran a 4.21 40, he maintained that average pace for all 40 yards. Every second, he covered exactly 9.5 yards...no more, no less.
For that matter, some guys don't maintain speed better going from the Underwear Olympics to pads....every analyst and scout who's ever said so is wrong.
Basically, "real" football is played in shorts, guys run in straight lines, and everybody is always moving at their absolute top-rated speed as measured at the combine over 40 yards. And the faster guy
always burns the slower guy, no matter what.
"Facts"