Because someone may not be aware of that fact, and may want to read through a much more extensive thread on that topic they may not be aware of. This generally happens to newer people, and they sit and wonder why this bunch of snobs don't chime in on their topics - all the while its because they already responded at length elsewhere on the same topic.
But I guess you can just ignore people - that'll learn 'em!
Of course, when the thread is almost exactly two lines down from the one you started... well, then I just do it to be a smart ass. Do you mind?
Peter.