Why should dolphins and elephants be sapient? They don't use technology, don't build cities.
. The greatest human genius in the universe couldn't hold a hammer without opposable thumbs, or light a fire underwater. It's deeply species-centric to assume that the only way to be intelligent is to build things. Really, a science fiction fan should know better. Do Hortas build cities? Do incorporeal life forms carry iPods? Intelligence is about the mind, not the shape and capacity of the body.
Science has discovered evidence of high intelligence (and yes, in some cases, tool use) among a variety of other life forms on Earth: great apes, cetaceans, elephants, corvid and other birds, octopus and squid. And many scientists now think that most higher animals are self-aware to a greater extent than we have traditionally believed.
Even if they were sapient yet eschewing technology, what would prevent them from communicating with Humans and starting a civilisation of their own, or joining ours?
It's a conceit of science fiction that interspecies communication is easy, because it's convenient for storytelling. It would be deeply foolish to assume that a fictional convenience would be true in the real world. Fictional aliens can usually pronounce phonetic speech the same way humans can, or else have magic technology to do it for them. In real life, different species usually don't have the ability to produce the same sounds, which impairs communication. But we know for a fact that dolphins and great apes are able to understand human speech, and that apes can communicate with us intelligently through sign language. There's even documented evidence of dolphins and elephants mimicking human speech to the best of their abilities. Of course, certain birds can mimic human speech perfectly, and there's research suggesting that they understand what they're saying to some extent. If anything, the problem is on our end -- they can understand what we're saying while we still struggle to figure out what they're saying unless they have the ability to communicate in our terms. Which hardly argues that we're
the more intelligent ones.
Another conceit of fiction is that alien intelligences would think just like humans so that mutual comprehension would be easy. That's another convenience of fiction that no reasonable person should expect to be true in reality. Even among humans, we often have great difficulty understanding people whose worldviews and ways of thinking are different from our own. Different species could have radically different ways of thinking and perceiving that could create enormous barriers to communication and understanding. For instance, there's abundant evidence of advanced cognitive ability in octopus, but unlike most intelligent animals including us, they're more solitary than social. So the factors that drove the evolution of their intelligence would be different from ours, and the way their intelligence works could be very different. That could create quite a barrier to understanding how they think. But of course any SF fan should know better than to assume that just because something doesn't think like a human, that means it doesn't think at all.