Well it's not the same if the character is entirely designed around the concept. Often times that leads to it feeling rushed or fake. As if the character was a puppet controlled exclusively by that fetish. It feels much more organic when the character has the option of either or, because then it truly feels like you're either submitting to them or bending them to your will.
The thing is, it needs to be obvious from the initial content that the character is designed for that. I mean, taking Evelyn for instance - this is a character that's been in-game now for two years. Her fanbase is composed of people who like her as she is. If you were to swerve her character into one that's broken down into a sub and make that the only path (obviously not whats happening - just an example) saying that that's what was always planned wouldn't fly at all.
You need to have some indicator of where the path is going with the initial content or you're just inviting hurt feelings all around. Compartmentalization of characters into different fetishes may feel less organic, but it also sidesteps several other problems.
That and the game is already EXTREMELY bloated with characters. We don't need to introduce a new one for every single niche and it's more efficient to give it to an existing character if possible. I see no reason why you should put it on another character when existing ones work just fine for it.
As you can see repeatedly from this thread alone though, people tend to get real mad even at the
option of a character doing stuff they don't like.