It's so they can establish a pattern down the road if necessary. It's obviously not a big deal if two people get sick on a ride over the space of a few months.
But if people are repeatedly getting ill on a ride, then there may be a connection -- but there's no way to know if people are repeatedly getting ill unless all illnesses are reported.
Same for injuries. Obviously, a 75-year-old woman who breaks her ankle on the Peter Pan moving platform is sad... but there's a good chance her age/condition played a role in it. But again, unless they report all injuries, there's no way of knowing if the Peter Pan platform snaps 2 ankles a week and therefore may be at fault... and so on.