I can’t think of a card-driven tableau builder that doesn’t have unbalanced cards, whether Ark Nova, Underwater Cities, Terraforming Mars, Everdell, or Hallertau—some cards are simply better than others. As for the why of it, the emphasis in these sorts of games is on each card being relatively unique, so that each draw is an event of its own, not just a random event but a lucky one.
As for the cards you chose, most players would feel lucky to get either of those cards, as there are only 10 petting zoo animals in the deck, and each player wants not just one, but 3 of those 10 cards. So most would feel lucky to draw either of them, unbalanced or not.