Hi Punk
How do we know? It is a question of working the info through our minds and deciding for ourselves whether the proof stacks up.
Why do people stay with Watchtower when it is so obviously false? Many reasons.
On ex-witness I know saw a lot of what went on behind the scenes in the back room. This one wouldn't even treat a dog the way some bro and sis were being treated. The cruelty of the organization was obvious and that was why that person left.
Me I left because I could see that the Bible opposed the very basis of WT authority. I had been lied to in my Bible study.
But some people aren't really fussed about cruelty and integrity. Or they don't see the significance of it at the moment. Others don't persist in trying to understand the Bible. They feel it's duff and get on with their lives. Their choice.
My bottom line, ok you think I'm wrong. Tell you what, I'll be wrong with Jesus. He strikes me as being an extremely nice person.