Because under partners/GMs who compose pre-made stories, one or more of the following detriments tends to be present:[list][*]Pre-established characters, which strip me, your RP partner, of my right to design my own character as I see fit[*]Pre-established relationships, which strip me, your RP partner, of my right to decide how my character (also not of my design) perceives yours; whether their personalities clash or are compatible, for example, which basically decides where most of the conflict in an RP comes from. If you describe it as a romance RP then I have no choice in whether or not my character is sexually attracted to yours; if I don't fall in love with you then you'll accuse me of sabotaging your RP, or not reading its premise properly, or something along those lines.[*]Alluding (probably) to the fact that you're an RP railroader; after all, if you've already thought of my character and the story in which he's participating, all on my behalf, then it's not unreasonable to think that you already know how his story will end, too. The more pre-made plots your thread has in it, the madder you tend to be when I end up wanting to deviate from your narrow parameters for where a story is "allowed" to go.[*]Autonomy in general. There's an attitude of whether or not I match the exact persona which you want for this RP, as opposed to whether I'm a good writer, whether my character and his personality are compelling, etc. You're basically presenting a job opportunity and picking the person whose résumé best matches your "ideal" employee, instead of letting us use RP as a [i]creative outlet[/i] where we're supposed to be able to roleplay whoever we damn well want.[*]And if any of the above is somehow not true, then you're communicating poorly, since you're giving the [i]impression[/i] that they're true by using the exact same methods to search for RP partners as the egregious fuckheads who actually do all this.[/list]

The worst example of this, which happened to me personally, was when she propositioned me for a historical RP based around archaeology in ancient Egypt. She mentioned that she more or less wanted to recreate one of her favorite book series (red flag #1), which was some kind of pulpy adventure series where the snarky British girl and the snarky British guy fall in love while excavating tombs and deactivating traps and stuff. She mentioned romance as a "maybe," which I thought was fine, since that gave me the option not to fall in love with her, right? So I made my character and sent her the CS, but she took offense with the fact that I wanted to play a character in his fifties. Which is clearly too old for a woman in her twenties to fall in love with. (Fun fact: for the Victorian period that wasn't true at all. So her historical accuracy readings weren't even that great, for someone who purported to love history so damn much. Red flag #2.) I informed her I had no intentions of letting this be a romance story, since it was initially pitched to me as a historical RP, which is why I agreed to join it and seemed interested in it in the first place. Well, she pretended that was fine, and that she would write the thread and send me the link soon, but that was two months ago and I'm pretty sure it's not happening.

As a matter of principle, all these factors and more are why I never, ever describe characters in my 1x1 plot prompts; no gender, no feelings toward the other main character, and certainly no assignment of roles to myself and my partner. (I will, for example, let the partner choose whether she's the prisoner, or the guard with whom the prisoner develops an unlikely camaraderie.) Even the plot is left deliberately vague, as I never want to direct my partner's character's feelings or actions. I will say "Character A is in prison and character B is his/her guard" but I won't explain how they feel about that or what will result from it. I give the circumstance/premise without giving very much actual plot.

[b]TL;DR[/b] RPers appreciate creative freedom.