The answer is simple:
Because I told you to. Now go to your room.
If you'd like me to elaborate, I shall do so: think of the application process as your resumé/CV. You wouldn't expect a business to hire you without putting in an application, now would you? Well, we're not going to employ you as a roleplayer in our dream without some idea of what you're worth. An application tells us a lot of things; it tells us how good of a writer/roleplayer you are -- and yes, whatever you may think, both writing and roleplaying are deeply intertwined. Trust me, I write pseudo-professionally! It gives us an idea of whether or not you're here to roleplay or here to beat all of us.
Most important of all, your application tells us just how dedicated you are. We aren't saying you're a lousy roleplayer; we're saying we don't know, and we need you to tell us. You may think that a simple roleplay could give us an idea, but believe me when I say it can't.
So do me a favor, bite the bullet and write the damn thing. I churned out a 4,225 word biography for my character and finished my application in the space of one or two days. You can write a little teeny one in under an hour if you just get off your ass and do it, and you know it.
Go get 'em, Rocky.
IMPORTANT NOTE: We don't deal in favoritism. Listen, you were great in NJO/TJO/Dantooine/SWU, but we can't just presume that you're right for a high rank in our dream. We have different standards set by different people. Just because you were the High Councilman or a Jedi Master doesn't mean we want you to be one here. If you used to be an upstanding member of another community, or a full member vouches for you, we'll take that into consideration, but you won't be getting the same rank you had in whatever dream if you half-ass your application.