I have seen cases where people did search but jumped to a topic which looked close by title. They did not read entire topic and problem being discussed and simply posted off-topic questions.
May be seeing more number of people involved in old discussions encourages them to post there as such offtopic replies go to other site members participated in old thread. I can say this because many times before moderator team, a regular user in that thread replies requesting new user to start new topic.
Though I’m happy with split and merge options. I personally would prefer a confused new user to create new topic over bumping wrong thread. Reason - old threads may annoy people subscribed to that old thread only. While staff will always get a notification.