Its worth noting that the idea that great apes have learnt sign language is largely a fabrication by a single person, and nobody has ever been able to replicate this. All the communication has to be interpreted through that individual, and anyone else (including people that speak sign language) have confirmed that they're just making random hand motions in exchange for food
They don't have the dexterity to really sign properly
They don't have the dexterity to really sign properly