i can only speak for myself, not all the other people, and my reasoning is this:
what is the point in having categories of everyone posts any questions where they want. I am not interested in, nor know very much about sport, for example, therefore i avoid that category. If the categories i go in were filled with sport stuff i wouldn't want to come here anymore
if you post in the right category, then you are more likely to get a useful reply, for example, people who know stuff about law tend to hang out in the law category (ie buenchennio, ethel, stu dent)
if everybody posts all assorted stuff in body and soul, for example then the front page of that topic moves very quickly, and while you CAN look at subsequent pages, i dont think people tnd to, therefore a corrctly asked question in the right category might move off the page so fast it dosent get the "air time" and therefore any replies.
And lastly, its just better that way for the pedants/obsessive compulsives amongst us!