Not all of the water instantly vaporizes when you reach boiling point because the boiling point is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid. At this temperature, bubbles of vapor form within the liquid and rise to the surface, where they burst and release the vapor. However, the rate at which vapor bubbles form depends on the temperature and the pressure surrounding the liquid. At the boiling point, the rate at which vapor bubbles form is equal to the rate at which they rise to the surface and burst. This means that the liquid will not instantly vaporize when you reach boiling point.

In order to instantly vaporize all of the water, you would need to increase the temperature of the water above the boiling point. This would cause the vapor pressure of the water to exceed the surrounding pressure, and all of the water would instantly vaporize.