Also, even if a single smart chimp discovered complex language on its own, what good would it do? (S)he would also have to know another smarter-than-average chimp who was smart enough to pick it up, and then, the knowledge would have to spread to a larger group fairly soon in order to survive.
Chimps could be relatively close to the brink of learning complex language, needing only a relatively small push to get over it.
Your point actually reinforces ggreer's -- if a species is capable of an advantageous behaviour like complex language, eventually one of the species will figure out how to use it to gain an advantage, like the birds you refer to.
The idea that apes have this ability but none ever figured out how to use it in order to gain an advantage is akin to the idea that mice could fly to escape predators but just never got the "small push" to figure it out.
Why would you think complex language would be a net advantage for all apes? Also, assuming that it, there is no guarantee that that eventually is in the past. Maybe, if left on their own, chimps would make the transition in a century or so, or in 20k years, or never.
The idea at the time was "let's push them hard to see how far they can get". Was the answer a guaranteed yes? No, but neither was it a guaranteed no. After all, we know that the ape called Homo sapiens did learn the trick, and we also know how much of a difference pushing a human towards a goal can make (see for example the Polgar sisters or quite a few tennis players).
That's why I mentioned robins and blue tits. AFAIK, we don't know why robins discovered the bottle opening trick, but never managed to engrain it in their culture, while blue tits did.
In a similar vein, if (just throwing this out there off the top of my head) the last common ancestor of apes and humans was 4 million years ago, why haven't apes evolved speech and tool making in those 4 million years?
Different habitats lead to different selective pressures.
If the last common ancestor of apes and humans was 4 million years ago, why haven't apes lost all their body hair and developed the ability to run on hind legs?
Because they don't live on the open Savannah, of course.
Speech & advanced tool making is more complicated, but we can surmise similar ideas apply.
For example, maybe human pack endurance hunting in the Savannah increased the value of social structure and sophisticated communication.
Also, even if a single smart chimp discovered complex language on its own, what good would it do? (S)he would also have to know another smarter-than-average chimp who was smart enough to pick it up, and then, the knowledge would have to spread to a larger group fairly soon in order to survive.
Chimps could be relatively close to the brink of learning complex language, needing only a relatively small push to get over it.