I've gone back and forth on this quite a bit. My personal preference is much like yours, topics should stay mostly on track, and new completely separate things should be in their own topics. It makes it much easier to go back and find something later, and makes it easier for people to know what threads to take a deeper look at. The problem here is enforcement. Due to the nature of the community, moderators have only really existed to deal with spammers, and not the actual content being contributed by the users. So to split topics after they are created would require more manpower. The other issue is how easily this can be done. If people just type in the quick reply box at the bottom without using the quick reply/quote links on a post, their response will fit in chronologically but will not be threaded properly. This can lead to the wrong replies being split off. And then there is the issue of posts that contain on topic and off topic discussion in one reply. That becomes a manual edit and manual post creation to split properly.

It's just the way things have been here since the start. Off Topic was specifically created due to on topic discussions going way off track. So I guess it was natural for the off topic stuff to just evolve the same way. The only way that situation can change realistically is to ensure the posts are created in the right spot by the creator. Adding more moderators to try to straighten things up after the fact is likely to turn messy, and add work.