Interesting question!
When my wife and I got tattooed in the early 1970s, I think we just thought that "a tattoo is a tattoo." Like "ketchup" - generic. Similarly, I guess we thought that you go to a tattoo artist and get a tattoo, and it never occurred to us at that time that there might be good tattoos and bad tattoos. Granted, the situation was much different at that time. Most tattoo shops were in really bad parts of town, and were really scary places inhabited by really scary people. Honestly, we picked our tattooer because we knew someone who had gotten a tattoo from him, and his shop was the least scary place we could find. When it came right down to it, we just wanted to get tattoos. Quality never crossed our minds.
Today, there are so many artists, and so much information about tattooing. Every general interest article about "things to consider when you get a tattoo" advises people thinking to check out the artists portfolio, so there is really no excuse for getting a bad tattoo. But I wonder if some people still look at it like my wife and I did: They just want a tattoo, and the desire sort of blinds them. So they just find a shop, go in, and get a tattoo.
I also wonder if after people get a bad tattoo, perhaps they realize it, but don't want to admit that they made a mistake in artist selection.