By realism I mean laying eggs and not live births as it is now. And you are also not entirely correct on how zoos handle eggs. Most species eggs are left in the nest with the mother. It causes more issues hand raising and they try to avoid doing that. Especially for species they are trying to reintroduce into the wild, as they will not survive if hand raised.
Usually hand-rearing occurs with puppets so the young imprints on the physical form of its mother. I have witnessed this occurring with parrots, birds of prey, and flamingos. I have also taken part in the artificial incubation of kiwi and tuatara (the latter eggs were difficult to find because like most reptiles tuatara bury them). This occurs frequently even with animals that are due for release to the wild. Even in cases where birds are allowed to breed of their own accord zoos will usually create artificial nest-boxes that are easily accessible for the keepers; there are very few instances where birds are allowed to breed naturally. With many species of birds there's a good chance one or more of the young is neglected in favour of its stronger nestmates, for example; by removing the eggs zookeepers can avoid this occurring. Additionally it's common for parent birds to destroy and eat their own eggs at the slightest inconvenience, especially in mixed-species aviaries where there is always the risk, even if it is only perceived, of another, more threatening bird getting into the nest.
Then there are the big birds, such as ostriches; in the wild ostriches typically build their nests away from the main avenues of animal movement. In zoos this isn't generally possible in mixed species habitats. Due to the nature of parental care in ostriches if whichever parent is guarding the nest gets scared off by a bigger animal (and this isn't uncommon, either - nesting birds are far less forgiving of provocation) then the nest is effectively abandoned.
In any case, despite what you believe about how zoos handle bird and reptile breeding, adding an egg-laying function in-game just creates an unnecessary extra step between pregnancy and birth. On top of not being especially realistic, it doesn't create any new challenges, and doesn't add anything significant by way of gameplay features. The animals born in-game, whether they are birds, reptiles, or mammals, are not newborn, they appear as juveniles (and even then age is inconsistent between species, with some being pretty close to newborn, and others being closer to adolescence). My point is that in the same way you have to imagine that some time has passed between birth and the offspring's initial appearance in the habitat for any species in the game (which you do, because a newborn lion cub is nothing like the game's baby lion), it's just as easy to imagine that eggs were laid and offspring were reared for a while before the baby birds and reptiles appear in-game.