Because it's too much work for mods to do.. and there's only a couple that are active much. Plus it's good to keep threads open if anyone has any valuable information that could benefit the other users, even if the OP isn't active.
And try not to make useless posts (eg "Cool!" or "Really?" or "Ok"). However if a reply like that is necessary and can be that simple, it's ok. Judging from your posts you always have a lot to contribute so that shouldn't be too bad.