Mental afflictions are caused by a combination of genes and environment, but often require a specific configuration of both in order to manifest. Properly changing the environment, then, even without actually changing the genes, would, in most cases, completely eliminate a given "disorder" -- so why, after having developed some arbitrarily defined mental "illness," are we taught that our thoughts are irrational, rather than that there is a mismatch between our genes and our environment?
Simple: because psychiatrists would be out of jobs if there were no more patients to retroactively treat
Imagine a world where, instead of abolishing slavery outright, we felt like we were doing the right thing by teaching American blacks that all that matters is that they're "cured" of their slavery -- while completely ignoring the responsible system. Instead of stopping slavery at its source, we'd "treat" slaves -- the "patients" -- by setting up slave-freeing services, which would require a small fee for our efforts. "Hey, I can get you off of this plantation. Just sign right here, and we'll have someone come by tonight to pick you up. Who's your insurance company?" In such a world, we could make ourselves feel good for freeing finite numbers of slaves while simultaneously making tons of money by doing so, because there would always be more slaves being born into the system to treat after the fact.
Metaphorically speaking, the solution to mental disorders is to abolish slavery (and to educate the population about its adverse effects). Literally speaking, it's to establish an entirely new societal system.
Mental disorders are not unique in this regard, of course. The underlying problem, itself both fundamental and abstract in nature, also manifests in the forms of: police forces, charity organizations, political lobbyists...