Because the chemical in this case is something we have all the necessary things to produce.
I can't make energy (ie. food) from no input, but with some energy expenditure, my body could carry out the set of reactions to convert glucose (which is readily available in the diet) to Vitamin C.
I can't make energy (ie. food) from no input, but with some energy expenditure, my body could carry out the set of reactions to convert glucose (which is readily available in the diet) to Vitamin C.