And why do you think they need to bring that many replacement units? ;-p
In my simple way if thinking, I would say that it is because:
1. Their camera got hit by a player that was rushing to try and get the ball before it goes out of the field. I see this often in football matches.
2. The cameras and lenses need to be used under harsh conditions, no matter what.
3. Shit happens.
Anyway, if I were a sports pro assigned to such an event, I would be glad I had that kind of support.
It would be interesting to know how many cameras and lenses Canon and Nikon need to provide as replacement units during such large events.