We are living in an era of caution. The brain is not fully understood and I feel the brain will soon have its day in the sun the same way the heart did in the 80s/90s. You would be right that we would need to adjust our baseline, however, as it stands the medication rate is 25% and not 80%. I wouldn't be willing to make the argument that we do in fact over-medicate because I am not an academic nor a psychiatrist, but my intuition from the media and what I've seen would tell me we do. However, the brain is an organ and disease emerges in it the same way it does in any other organ. Why should the brain somehow be more resistant to disease than, say, our heart? Is it not too subject to the consequences of lifestyle choices, diet, genetics, etc. ? We understand mental illnesses far better today than we did a couple of decades ago and the disease prevalence has increased because we know what is a disease and what is something else. I would say that 20% number is probably very accurate (if not generous).
Think of this way:
Imagine that you lived (as I do) in an extremely fit city in a very obese state. The NIH tells me that more than 30% of my state's population is obese. I would look around my city and laugh and say, "Oh please, 30%??? Look around!"
Now imagine instead of a city it is your mind, an even more private thing than a city and a smaller sample set to boot. Would you look at your own mind and say, I'm perfectly fine - why isn't everyone else?