Among the requirements to donate blood is the weight of not less than 50 kg; I cannot donate because, although I am in good health and exercise, I weigh less. How correct is it to rely only on your weight and not on your general state of health?

The minimum requirement has nothing to with the patient’s is determined by a percentage calculation to avoid the risk of hypovolemic shock, i.e. reducing the amount of fluid in the in a short time.
For legal limits it is possible to draw to 495 ml of blood from a person. According to current estimates the amount of blood circulating in a healthy individual is 7% of body weight, in a person of 50 an average of 3.5 L of blood circulates. It has been estimated that if 495 ml were taken from a patient with a lower weight, having blood circulating exceed the safety to avoid shock.
It is a matter of safety for the donor.