PVC in itself is a white brittle plastic. You wouldn't believe the number of additives it needs before it becomes suitable for the many purposes it's used for, like underground drains, windows and doors, cable sheathing. waterproof clothing...the list goes on and on. But additives cost money, so manufacturers will look at the durability needed for their finished product before including them. Flowerpots would be looked upon as being fairly expendable. Window frames have to me made to last.