It's not in the government's best interest to stand between people--especially those who are desperately ill--and their desire to take particular medicines. But from a science-based standpoint, patients do not know what drug treatment will and will not work.
Doctors thought for years they could tell what worked. In the 1960s, for example, doctors were convinced that diethylstilbestrol, or DES, was terrific for preventing early miscarriages, and they gave it to thousands of women in pregnancy. When DES was actually subjected to scientific testing, it had no effect on miscarriages. Not only was it absolutely ineffective, but unfortunately, it had delayed negative health effects on the fetus.
Many smart people have thought they knew what drugs would help them and what drugs would hurt them, and clinical tests again and again have proven them wrong. Without testing, we didn't know.