I know that and you know that. But try convincing a Jehovah's Witness of that, especially when the word can be found nowhere, and there are passages that seem to say the opposite ("my Father is greater than me"). Most everyone who argues against the Trinity does so from Scripture.
Scripture is not as glaringly obvious as many people think it is.
The JWs do not argue from the scripture necessarily. They remake scripture to fit their purposes by such as John 1:1, making the Word "a god," for no legitimate reason. When it comes to cultists, or anyone really, the problem isn't a lack of facts, which can always be provided even when they lack them, it is spiritual darkness that only the Holy Spirit can break through. We can only do our part by telling the truth, which always comes to our side.