Pretty much every tattooed person I have ever met got a tattoo because they liked it, and they thought it would look good on their body.
Also, lots of people get tattoos that carry deep personal meaning. One of the most affecting ones I’ve seen are breast cancer survivors who get tattoos like this:
How can anyone criticise the choice of these women to reclaim their own bodies by decorating them in the way they see fit?
BESIDES, fun extra, it’s none of anyone else’s business. There’s an absurdly judgmental other answer whose author seems to assume that the women he is slating care about his opinion of their bodies – I’m fairly certain they don’t.
I guess the short answer to your question is: because they want to and they can.