I think those long-winded questions are also frequently followed-up with frustrated "why isn't anyone reading ALL of my question and answering ALL of the questions in it?". Writing concisely and clearly is more challenging than rambling questions. in my opinion that challenge is what separates short posts from long posts.
I can see two reasons for it: - They're new on the site and they don't know that most people won't read the details. -They're desperate or worried and they're looking for genuine help, they tell everything so other people can help them.