I'm sure there are a lot of fine people out there who do these things to themselves, but from first hand experience a lot of them were not friendly to others (including me), or were involved in criminal activities(on occasion towards me). This is mainly bad experiences talking. While piercings have become acceptable (unless you look like a pin cushion instead of a human being), tattoos are still used for showing that you are part of a gang, or sometimes brag about things that people generally aren't proud of. That said, there are some sexy tattoos, and they look good on some people, but I would not put one on me.

I look forward to meeting some one who has piercings and tattoos who will break this image, hopefully soon.

To be fair, wearing the wrong color hoodie in the wrong neighborhood can be equally detrimental.

Also I think a lot of the reason people do body mods is because of rarity.
while gold and diamonds may be a bad example since they have industrial use, some people think they are cool to wear. I mean they could wear other gems or metals. You could get a normal piercing or henna tattoo.