On another thread (about dolphins finding ways to eat octopi without getting suffocated by still-alive-and-fighting disembodied tentacles), some similar thoughts came up.
I have to wonder if one reason for slow progress is that we've bored our subjects. Maybe we should send poets and musicians as ambassadors, not literal-minded scientists.
I read of a researcher who played recorded dolphin sounds for dolphins, and she wanted to teach dolphins to say names she made up for three objects. I don't think the results were very encouraging. But that might be because the dolphins found it pointless.
Dolphins are aware of one another's sonar. So, they would never need to vocalize "Look at that over there." They already know what pod-mates are paying attention to. Why would they verbalize about things that are already obvious on the pod's echolocation radar? They might not have a use for concrete nouns. Dolphins' vocal communications might deal with abstract things like social dynamics, emotions, intuition, and humor.
One other clue: I read of a recording of an upset, irate dolphin. When slowed down significantly, it was clear the dolphin was mimicking it's trainer's reprimands. So, we have to start by admitting that their processing speed is way higher than ours. They're also the only animal with a more highly folded cortex than ours.
These are creatures that can be taught a command that means "innovate" - do something different that you haven't shown me yet today. Maybe innovation is a key aspect of their language. Maybe we need to think way outside the box about what a language could be.
As for dolphins and building ... they don't need protection from sun, rain, and snow; vast swaths of ocean are just the right temperature for them; they can sleep with half their brains at a time while swimming; and they are not interested in stockpiling food for later. What do they stand to gain from building?
What dolphins lack in material culture, they might make up for in social life, poetry, philosophy, and music. Maybe they're looking at us with pity: "Their pod dynamics wouldn't be such a mess if only they didn't waste so much time and effort building stuff..."