why don't I leave ?
You don't want to leave/ you are not ready to leave. That is the answer to that question.
Perhaps you think you can still save him.
Perhaps, like so many others here, you are romancing the notion that your love will turn this around.
His statement of being "done" with you is a classic example of protecting his addiction, your are now interfering with drinking, and he will banish you from his kingdom, as he will not allow anyone to compromise his right to drink.
The fear of being alone can keep us stuck, what we fail to realize, being with an active alcoholic, we are already alone.