Hebrews 1: 1 - 2 God, who at various times and in diverse ways spoke long ago to the fathers through the prophets, has in these last days spoken to us by His Son
The reason the WT exists is because of biblical ignorance. No one is God's channel in the Church Age as the verse above announces. The Son inhabits each saved believer through the new birth.
Col 1:27 To them God has chosen to make known among the Gentiles the glorious riches of this mystery, which is Christ in you, the hope of glory.
Jehovah's Witnesses (and all other unsaved people) are naturally quite happy with their first birth and certainly don't want Jesus living inside of them. So, they opt for another channel since the one God provided isn't good enough for them.
The reason that God must work from the inside is because exterior Laws were proven beyond a doubt not to work in the Age of Law, which ended at the Cross.
Christ was the end of the Law. He offers himself not only as a substitute for our punishment due each of us, but also as a substitute for our "righteousness". Is. 64: 6 - "all our righteousnesses are like filthy rags".
If people had a clear understanding of what Christianity is, regardless if they personally accept it or not, the WT could not exist because it is so opposite of scripture and apostolic teaching.
If people make the choice not to invite Jesus into their being, I can't see how they could ever follow such a blasphemous organization such as the WT. They only do it out of ignorance.