"Perhaps, Nadeau reasoned, the problem lay in the sperm, not the egg. He therefore bred male mice with and without the mutation to healthy mutation-free females and found no differences in the males’ fertility"
Of course scientists are trying to identify the underlying causes of mental illnesses. They're even making a lot of progress, but the brain is just unbelievably complex.
In the meantime, treatments are found by trial and error. Testing whether a drug treats a disease's symptoms can be scientific even if you don't know how the drug actually works.