seem to recall something like 30% of the adult American population being in therapy or having been recently.
My impression is that mostly involves people going to their doctor and saying "Doctor, I feel horrible!". And the good doctor says "Sure, try these antidepressants!" (yes, I know I'm exaggerating).
That's a different thing from "Doctor, I believe I'm mentally ill".
Depression is a mental illness. You might not go to the doctor and ask about depression (though I doubt this is anywhere near as uncommon as you're making it out to be), but going to the doctor and saying "Doc, I can't sleep, feel sad all the time, everything I do seems pointless, etc." is as much asking for a consultation on mental illness as "Doc, I've got this nasty bullseye-shaped rash on my leg and I've got a fever and a bad headache" is asking for a consultation on Lyme disease.
The standards of diagnosis might not be as rigorous, but that's a separate issue.
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