The ONLY thing which prevents chimps and gorillas from speaking English is the lack of voluntary control over breathing. They learn deaf signs easily.
It's not as simple as that.
They can learn signs, but their usage is much more basic than human language.
A lot of Koko's usage was more of a very elaborate Clever Hans type of system. And a LOT of what she "said" had to be interpreted by her
owners. It was largely difficult for anyone else to independently understand her signs, or to communicate back to her.
Hey, it's still pretty amazing, but English language it is not.
Also, many primates do have SOME voluntary control over their breathing, because they can also be trained to play wind instruments such as recorders.
So, chimps and gorillas may be smarter than horses. But, it would take a LOT more than merely better control over breathing to allow them to speak English.
Humans innately develop new languages, in a manner of a couple of generations, when there is none around they can use already: Such as deaf children in schools before sign language was standardized. Or, the pidgin languages that develop, when there are people working together who only speak a multitude of incompatible languages.
If other primates also evolved this ability, we would see it in the wild as an innate characteristic, even if it took a different form than speech.