How “mental illnesses” are described and defined in scientific literature changes as science (and bureaucracy) progresses, as do our methods of measurement, and our scientific understanding of what is involved- science moves faster than languages evolve to adapt to its progress.

It is therefore not strictly and necessarily a matter of increased prevalence, but how we encapsulate and express these things culturally.

Compare the logical conundrum of the infinitely expanding shoreline. Was it Alfred Korszybski with that? My memory fails me.