Too much Novacaine etc can make the skin go all mushy which makes it more difficult to tattoo and heal, which increases the chances of it looking crap. Instead, people with money and aversion to pain get put under general euthanasia while a team of artists all tattoo at the same time. The only real downside is that general anaesthesia is always dangerous so it’s probably too big a risk just to avoid a few hours of pain and discomfort