Answer: Because shammash doesn't.
Both Eris and shammash came to us as strays. Eris was skulking around by a filling station in Southside Phoenix (a bad neighborhood for anyone, much less a six month girl-cat). shammash was sitting in a patch of sunlight under a bush when votania was walking somewhere. He was crying.
Eris is just skinny as hell. Someone in her past must have been nice to her, because she likes humans and snuggles up. She was dirty and still has ear mites, but is otherwise intact.
shammash had things go rougher for him. He only has half his tail. When we got him, we noticed with some horror that his tail had at some point been badly hurt, leaving him with a crooked-ended stump. It was healed when we got him, but it still bothers him when it is touched.
We're not used to cats with long tails around the house. 'Mash is very expressive with his 1/3 to 1/2 tail, though we miss the refinements that more fully-endowed cats add in their tail-language.