A:  Because mind-based problems generally involve localized sets of highly compartmentalized molecules, small numbers of cell cohorts, isolated subsets of neurotransmitters and neurotransmitter receptors, &/or precisely specialized network connections.  Our knowledge & understanding even of the "normal" mind, although growing rapidly is still crudely limited.  And the psychoactive drugs currently at our disposal are far from selective, not being refined enough to exert the kind of selective, localized influences that would minimize unwanted side-effects.  These drugs are far too crude & invariant, doing far too much to too much brain tissue & doing it all the time, at full throttle, like the car whose cruise control mechanism is stuck on 100 kph.