Why do tattoo artists dislike rib tattoos

It may be because:

Clients who have never gotten a tattoo before like the way a rib tattoo looks, but don't have experience with the feeling to gauge whether they can tolerate the pain. 

The ribs/sides of the torso are a super sensitive area and the client is very likely to flinch or make a sudden movement, which can cause unexpected mistakes. Even for people who have tattoos in other areas of their bodies, their reference for how it will feel may be based off of the pain of the needle in less sensitive areas.

The client may not be able to tolerate the pain and may choose to“tap out,”(stop the session) leaving an unfinished tattoo. 

The ribs are a bumpy area where skin is directly over bone, with not much muscle or fat in between to even out the surface, again causing more pain than other places on the body, but also becomes more awkward to work out larger designs.

Rib tattoos are not as likely to be turned down by the artist as much as a hand tattoo or face tattoo would be. (This is because the latter are ill advised due to those areas being impossible to hide; these are known as "job killers")

However, they are not recommended for first time tattoos for the reasons I listed above. Be aware of what you're signing up for, and be prepared for the experience to be more intense than you imagined, once it starts.