There are many remnants of the business world left in JW terminology, from both Russell the wannabe businessman and Rutherford the lawyer and wannabe judge. Just like publisher, governing body has more secular origins.
But you're right: they claim to be directly appointed and 'sent forth' from God and Jesus themselves, so technically there should be no issue with being called apostles. However not doing so makes it easier to talk out of both sides of their mouths. They also have written lots of material denouncing apostolic succession which would have to be backpedaled. And besides, the Mormons beat them to it with their governing body, "The Quorum of the Twelve Apostles".