Because they like to make stuff more than they value a subscription. I'm gonna write music no matter what happens to it.
What are you going to do with it? If you publish it, the law currently allows Google to hoover it up and there's nothing you can do about it.
In that instance, google loses value over time because less and less valuable content is published because there's no point because people may read it as an AI summary but probably aren't going to share their own findings or discuss with you anyway.