That ExecutorService has actually two shutdown-related methods; based on the simple fact that both ways of shutting down a service make sense.
Thus: how would you auto-close a service then? In a consistent manner that works for everybody?!
So, the reasonable explanation in my eyes: you can’t make an ExecutorService a AutoClosable because that service does not have a single “close” like operation; but two!
And if you think you could make good use of such an auto-closing service, writing up your own implementation using “delegation” would be a 5 minute thing! Or probably 10 minutes, because you would create one version calling shutdown() as close operation; and one that does shutdownNow() instead.