Why do cows have to live on high land?

They don't have to, really. Depending on the breed, some breeds like living up on the hill tops whereas others prefer the bottoms. It's more down to preference than anything, there's nothing that says they have to live on high land, even in flood-prone areas. If there's a flood, well then the animals are smart enough to move up to higher ground, they don't need a farmer to force them to go to higher ground.