why does what other people do make something so personal fundamentally less cool?

When a significant amount of people do something, it loses it's "cool" factor almost invariably. Instead of being "cool", as in novel, original, unique, etc., it becomes mundane, "normal", status quo. Sometimes quickly - think weeks or months - and other times more slowly.

To give an example of the former: Harlem Shake. When it first hit the internet, it went viral, most people though it was at least amusing and original. Within a matter of DAYS everyone and their brother was doing it in some shape or form, and it quickly became old and tired.

An example of the latter: tattoos. For a long time tattoos were a clear indicator that you were on the "outside" of mainstream society. Criminals, sailors, vagabonds, etc. Members of mainstream society who were less "fringe" but still "outsiders" started getting tattoos, and while still frowned upon in true mainstream society, it not longer necessarily meant you were an unsavory character.

Fast forward decades and we are not at a point where tattoos are extremely common place. It's not at all strange for someone to get a tattoo, or three, or ten. Even in the corporate world tattoos are slowly becoming accepted.

You can argue whether or not this has "ruined" tattoos (per the OP), but there is no denying that tattoos are far less "cool" and interesting than they used to be. Also - and this is just an opinion - the "tattoos are so personal" argument falls pretty flat these days. A lot more people get tattoos not for personal reasons these days, but to fit in, or fit a certain aesthetic.

In my own circle, I'd say 80% of people have tattoos, and at least half of those are fully sleeved etc. It's actually more surprising when someone doesn't have tattoos.