This right here.
Ideas don't need to map to reality(see: political discourse). Some ideas are constructed with a fantastical foundation (ie magic, anthros, many kinds of vore, growth/inflation) that precludes all but a proximal representation of them in the real world such as art, stories, erotic roleplay, or costumes.
Another subset of ideas are built upon real phenomena, but the experiences and sensations surrounding them are unknown, and so are filled in with the brain's best guesses. One example is death. Fictional and real living things can easily effect our emotions differently. A person playing a tabletop or video game might encounter the death of a large number of humanoids and be almost completely unaffected. If the same person were to watch real footage of a human being killed, they will likely map that to the concept of a real human dying(not always) and be emotionally effected. Most extremely, if a person directly witnesses the death of many people, it is very likely they will develop PTSD. I believe this would apply to the overwhelming majority of people attracted to snuff as well as fans of violent media.
Lastly, there's a subset of ideas that map directly to reality and can be imagined by drawing from real experiences(vanilla, anal, bodily function kinks, prop kinks, bestiality, and ageplay). This subset is likely the whole point of this thread. The same explanation for liking these only in fantasy described above can apply here as well. Your particular conception of anal doesn't need to map to real experiences, even if it is possible. (see: painfully-large insertions) Even if you know the reality of a kink like castration would be significantly unappealing, there is nothing stopping you from exploring a fantasy involving it and cherry picking the aspects you want to focus on. Overwhelmingly, the effort of creating media containing a specific kink will not be wasted on the less-desired portions of it. Notice that the picture in the OP doesn't show the cage being installed, the piercing being made (and the consequent bleeding/healing/infection), It's either too expensive to do so, undesired, or both.