There is no silver bullet for any kind of problem in software engineering. And, distributed systems is no exception. Also, when designing a large system e.g. a distributed system, the fact of selecting a language for its functional, procedural or object oriented capabilities is not much important afaik.

Hence, I think the first sentence of your question has no effect on the question. For the question of >why isn't it functional? I think current state of Go is performing enough for Google's problems.