Because psychiatric drugs don't necessarily work by counteracting specific deficits in the brain. That's a myth driven by unfortunate marketing campaigns and surface-level misunderstandings of the science.
This makes intuitive sense to people when discussing other topics. For example, when someone is in pain they don't need to have an "endogenous opioid deficiency" for opioid painkillers to alleviate their pain. They're just in pain, and modulating the opioid system with drugs is one way we can reduce that pain. Or COX-2 inhibitors. Or a number of other substances.
Likewise, modulating the serotonin system with SSRIs is one way we can modulate depression symptoms. People don't need to have a "serotonin deficiency" to see benefits from modulating their serotonin system.
This makes intuitive sense to people when discussing other topics. For example, when someone is in pain they don't need to have an "endogenous opioid deficiency" for opioid painkillers to alleviate their pain. They're just in pain, and modulating the opioid system with drugs is one way we can reduce that pain. Or COX-2 inhibitors. Or a number of other substances.
Likewise, modulating the serotonin system with SSRIs is one way we can modulate depression symptoms. People don't need to have a "serotonin deficiency" to see benefits from modulating their serotonin system.