For the sake of this discussion, mental illness includes ADHD, OCD, bipolar, anxiety disorder. Open to discussing schizophrenia and other disorders, but I'm not so confident these are fake.
The primary "tells" for ADHD are inattention, hyperactivity, and impulsiveness.
The dawn of phones, gaming, and TV brings the above behaviors in spades. As these behaviors increase, it's only natural that ADHD diagnoses increase (significantly).
So, are ADHD diagnoses on the rise because these behaviors are more common, or because ADHD is more common? To answer that, we need to answer the following: is ADHD a genuine brain disorder, a condition which the patient has no control over? I argue no; ADHD is society's way of explaining the three mentioned behaviors without claiming responsibility. There's no concrete cause of ADHD, and the diagnosis simply tests for the behaviors. ADHD is, by definition, the three behaviors manifest.
Likewise, I'm willing to say that bipolar disorder, anxiety disorder, OCD, are all products of extreme behaviors, not an actual issue with the brain.
I'd love to hear your input.