I like to think of mental illness like myopia. Many people used to be blind, but then we figured out the technology, it got cheap, and now 60% of adults wear glasses.

As there is an incentive to classify mental illness for insurance, occupation, and drug development reasons, there is going to be 60% of people on ‘mental illness’ treatment, because we have an ‘ideal human’ (20/20 analogy) that we are trying to reproduce.

As the ideal changes (e.g. homosexuality was thought of in the past as non-ideal, so it was a mental illness, now these things have changed in many parts of the world, so those parts of the world do not classify it as such anymore.), the treatment options/norms change.

tl;dr, it is entirely possible due to biology that eventually ~60% of us will have some classification of mental illness that we will be treated for.