> it should be the first athlete to ENTIRELY cross the line
No, because then you'd be changing the distance of the race.
Runners start behind the starting line. So for consistency, they finish when the first part of their torso passes the finish line.
Your suggestion would only make sense if they started with e.g. their trailing foot touching the starting line and the rest of their body in front of it. But that's not how they start.
Also, if you based it on foot position rather than torso position you'd be introducing a rather random element, since it would change based on what part of their stride they were in. Torso position is much more continuous, and therefore is a far more accurate assessment of athletic ability.
Do they start with their head behind the line? If so that's really running 100m + 1 head. The distance is completely arbitrary anyway so it doesn't really matter.
No, because then you'd be changing the distance of the race.
Runners start behind the starting line. So for consistency, they finish when the first part of their torso passes the finish line.
Your suggestion would only make sense if they started with e.g. their trailing foot touching the starting line and the rest of their body in front of it. But that's not how they start.
Also, if you based it on foot position rather than torso position you'd be introducing a rather random element, since it would change based on what part of their stride they were in. Torso position is much more continuous, and therefore is a far more accurate assessment of athletic ability.