Here is the harsh reality .
Calves that are sickly or weak are abandoned by their mothers.
The park services do not remove them and save them because they are food for the predators that would otherwise take healthy calves .
You’re right.
I am a huge animal person (not a PETA nut) and I have followed a million stories of animals who were cast off by the mother, rescued by humans, and the humans are shocked when the animal dies of some unanticipated malady.
Knut, the polar bear in the zoo in Germany comes to mind as a prime example. Some years ago, he was born, rejected by his mother and preserved by the zoo staff. He lived in an exhibit with his mother and sister, who wanted nothing to do with him. The media got hold of him, and he became a rockstar, with huge crowds coming to see him. It was a big draw for the zoo.
Turned out he had some kind of circulatory system defect, he popped a blood vessel in his brain and drowned in front of a horrified crowd.
This is but one story of many I have followed. I have no idea how animal mothers know so early on that one of their offspring is doomed- but they sure seem to.