I think it’s because James is using only 18 m of wood, and only whole numbers. With only whole numbers, there are no more factors of 18 that would make sense for a picture frame, and the maximum area has been found with the dimensions of 6 and 3. If he continues the chart and goes higher than 20 (let alone 18), it would be pointless since he wants to make a frame with 18 m of wood.
