I roleplay because I like stories. 
And a story can be boring if you know what's going to happen next.
But a Roleplay is a story that has no conclusion, you don't know what's going to happen next, because the other people roleplaying with you don't know what you're going to do next. And the GM, who has the best idea of what they're going to present to the players next, has no clue how they'll respond to it. It's this uncertainty that is so delightful. An RP is constantly changing, with different people acting and reacting off each other. 
Writing a story requires you to know everything that will happen next. Reading a book, or interacting with other media similar, (films, ordinary plays, videogames) is an ultimately fixed outcome. The book ends, the play finishes, the movie goes to credits. Even videogames, which herald interactivity, aren't totally interactive. All you're doing is inputting responses into a static system of logarithms, and we trick ourselves into believing that it's fluid. When, in reality, the 'hyper-openworld sandbox RPG with more possible endings than atoms in the known universe' only lets you do what the programmers thought you would want to do. Fable is a great example of this: it claims that you can do anything you want, and be whoever you want to be, but really you can only do and be whatever the game is coded for. 

A roleplay isn't like that. Because a roleplay is a story that isn't finished being told. You can't know the entire story, because the storytellers don't know it either. And that's why it's okay that roleplays get abandoned. Sure, it's sad, and you'd like to know how the story concluded, but that's not the point of the story. The point of the story is to have fun.

I have no clue where I wanted to take this rant, so I'll close it here.