Awe-inspiring animals such as giraffes and gorillas and cheetahs and chimpanzees are not seen as individuals, with distinct perspectives, when viewed, as Dick says, as either useful or useless "specimens." They are valued, if at all, as representative carriers of their species' genes.
This distorts our understanding of other animals and our relationships to them. Part of the problem is that zoos are not places in which animals can be seen as dignified. Zoos are designed to satisfy human interests and desires, even though they largely fail at this. A trip to the zoo creates a relationship in which the observer, often a child, has a feeling of dominant distance over the animals being looked at. It is hard to respect and admire a being that is captive in every respect and viewed as a disposable specimen, one who can be killed to satisfy a mission that is hard for the zoo-going public to fully understand, let alone endorse.
Causing death is what zoos do. It is not all that they do, but it is a big part of what happens at zoos, even if this is usually hidden from the public. Zoos are institutions that not only purposely kill animals, they are also places that in holding certain animals captive, shorten their lives. Some animals, such as elephants and orca whales, cannot thrive in captivity and holding them in zoos and aquaria causes them to die prematurely.