If wheat, rice, potatoes, and tons of other types of carbohydrates are all made of the same compound, starch, which is (C6H10O5)n +(H2O), then why do they taste different and take different shapes and grow in different ways?
They are from different plants, potatoes very different from the grains, but they all are storing usable nutrients for the future, for the following year or the following generation. Those nutritional needs happen to be very similar, which is interesting: there likely are very different qualities as well. Starch is a stable storage mechanism and is not readily digested, maybe a chemist could explain better but starch requires cooking, which is something humans are good at.