I think you could probably say the same thing about a lot of activities: how is jumping out of a plane fun? How is sitting in a field waiting to take a picture of a bird fun? How is blindly manipulating symbols in a computer fun? You can play that game for anything. But for every activity someone enjoys, you can bet that it's not the reductio ad absurdum restatement of that activity they find fun, it's something else about it.

It's odd to be in the position of defending trophy hunters, so that's not what I'll do. There are many defensible types of hunting, but hunting rare or endangered animals for nothing else but a trophy is ethically indefensible. However, for people who do that, I imagine it's not the ending of the life that's fun, it's the challenge, and especially the bragging rights it gives them afterwards. Just a guess though.