When you have a mixture of water and alcohol present, the liquid boils at ~78.2°C (172.76°F) (water boiling point 100°C (212°F), pure ethanol boiling point 78.5°C (172.76°F)).
As more and more of the alcohol is extracted, the boiling point of the liquid remaining in the boiler increases until at the end the liquid reaches a boiling point close to water.
This is why the ABV also decreases over time.