[BluffTHIS!]

Because it might be the case that the only beings God regards as "human", in the sense of having an immortal soul, are Adam and Eve and their descendants. That would exclude members of other species (which Neanderthals are regarded now unlike formerly when they were thought to be a subspecies of our own), and as far as we know possible members of our own species who lived around the time of Adam and Eve. Thus there could have been beings who were intelligent but they just lived and died and that was it. This would in fact be similar to the beliefs of some but not all of the Jewish Sadduccee sect who believed that none of us has a soul that survives death.

All of this is speculation, but Catholic theology teaches that it is possible that such was the case for some, and that that possibility cannot be denied as an article of faith (cf. Fundamentals of Catholic Dogma by Dr. Ludwig Ott). And the same could be said for possible intelligent life on other planets. That is, they were created by God either directly or indirectly through evolution, but God chose not to give them souls.

While religious believers might contend that having a soul not only implies intelligence, but also the reverse, the fact that other animals have at least the intelligence of a very young human child as in the case of dolphins, which have no place in the eternal life according to scripture, would likely imply otherwise.