Because overwhelmingly, women don't allege abuse when it hasn't taken place. You know why, because when they do, they get ripped to shreds, disbelieved, and their life becomes much, much worse - even when it's true. And, I've heard him say controlling, obsessive, and abusive things. People other than Amber Heard have been on the receiving end of his erratic, substance abusing, behavior. It's been something talked about in the press for years.
That doesn't mean that she doesn't also have issues, but that's not what this case is about.