I suspect the answer is similar to why a macaque can't play billiards even though it is physically capable of it.
It's not like macaque monkeys don't "speak" to each other, they do make sounds that indicate certain things that other monkeys understand. They just don't have the cognitive ability for language. It's too complex and abstract for them.
Similarly, they can manipulate a pool cue and even knock balls around a table with it potentially. But they can't actually understand the rules or play the game effectively, even though they can be taught rules of much more simple "games" like "press the red button and get a treat, press the blue button and get nothing."
It's not like macaque monkeys don't "speak" to each other, they do make sounds that indicate certain things that other monkeys understand. They just don't have the cognitive ability for language. It's too complex and abstract for them.
Similarly, they can manipulate a pool cue and even knock balls around a table with it potentially. But they can't actually understand the rules or play the game effectively, even though they can be taught rules of much more simple "games" like "press the red button and get a treat, press the blue button and get nothing."