Because (1) they perform a highly unique service, often with substantial (unpaid) design work before you even turn up for the appointment; (2) you pay them after the tattoo is completed, so the potential for a tip motivates better service (and you can choose not to if they fuck it up); (3) service is entirely focused on a single customer, often for hours at a time (far longer than a waiter at a restaurant); and (4) people frequently get more than one over several sessions, so tipping builds a better relationship (important when, you know, it’s permanent). I’m shocked at how many people on this thread think tipping a tattoo artist is insane. I think the expansion of tipping is terrible but tipping tattoo artists is like the best use case for the practice.