Because humanity is the only known intelligent species in the observable universe.
Please step outside and be amazed when realizing what the other species on this planet are capable of.
Well, other than possibly being catapulted to space in spore form by a big impact event.
Animals are awesome and I am indeed amazed by how smart some of the species are. Not a single one is near human level, though, and won't be for millions of years, if ever - which is not at all guaranteed, it's very much possible that human-level intelligence is evolutionary mistake/accident.
Let me know when you find an animal that can do e.g. lambda calculus and relational algebra like a human can. Since this has nothing to do with anthropocentrism, the same argument will be made - we have to preserve this species on another planet to ensure that intelligence doesn't disappear from the observable universe in case something happens to Earth/its biosphere.
"For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much --- the wheel, New York, wars, and so on --- whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man --- for precisely the same reasons."
And why would such disappearence be bad? Really, honest question. Is there some inherent greater good to adher to by preserving intelligence, no matter how narrow it's being defined in this thread?
It could just cease existing. I don't see the problem.