We don't seem to have an innate grasp of what makes for a good insulating material. I remember that when I bought my brick house in cold Seattle, it made sense to me that the bricks would hold in the warmth during the winter. This turned out to be a completely false assumption. Why doesn't the cooker keep on heating forever? Well, as the temperature rises, a larger and larger proportion of the heat leaks out through the walls and the glass. The temperature continues to rise until the amount of energy coming in equals the amount going out.