Because the neurotransmitters are all we know enough to experiment with so far.

Like the joke about the drunk looking for his keys only right next to the door - "because that's where the light is".

Psychiatry has convinced a lot of people that mental disorders are often caused by 'chemical imbalances' whatever that means.

The neural structure of the brain is very complex and involves dozens of neuronal molecules acting on a wide variety of neurons.

It's incredible that we have psychiatrists prescribing all sorts of inhibitors/activators to alleviate various disorders. A lot of these drugs have unknown mechanisms. Even when the mechanism is understood no one understands how these drugs are working on the whole brain.

Neuronal pathways are based on a specific neurotransmitters. Dopamine usually means limbic system and seratonin usually means the cortex.

The parent here makes a good point, and your comment appears to represent a common misconception about neurotransmitters. Both dopamine and serotonin can do different things depending on the system and the receptors that are actively present. Serotonin, for example, is mostly in your gut, not cortex! From wikipedia:

"Approximately 90% of the human body's total serotonin is located in the enterochromaffin cells in the GI tract, where it is used to regulate intestinal movements."

Dopamine is more specific, playing an important role in the limbic system, but is not limited to it. Its functions are not completely understood, and again depend on the receptor.

Better to think of the function of neurotransmitters as dependent on both system AND receptor. The function of neurotransmitters, like dopamine, can be simplified, but others, like glutamate, are extremely complicated.