Because uncontrolled and random "neuroplasticity" isn't beneficial like psychiatry marketing is trying to sell. Look on Tardive Dyskinesia - example of a drug induced persistent syndrome. There is a new post mortem research done on brains of Tardive Dyskinesia monkeys and that research found antipsychotic induced new connections in neurons of basal ganglia. New synapses are a brain way to balance against dopamine antagonist action. But these connections are aberrant ( perforated synapses ) and random which cause overall disruption in brain circuits controlling muscle movements. That's why symptoms appears. Other basic research on neuroplasticity found that drug induced neuroplasticity changes are very persistent and hard to extinct. These changes are made by a foreign intervention outside original brain homeostasis so nervous system simply doesn't have a feedback loop to control / destroy them. That explain why Tardive Dyskinesia is so persistent.
I think such neuroplasticity microstructural changes ( incorrect loss or gain of synapses and modification of brain circuits ) are true for long term use of all neurotransmitter acting drugs. Changes are diffuse and random. That's probably why PSSD ( if neuroplasticity changes from SSRI use develop into syndrome) has so many persistent symptoms and no simple treatment.