If I ask a question and it has been answered, I decided not to lock the thread because some other users may still want to contribute, I like all experienced members on this forum to contribute, although my thread is not a spam thread and I always prefer to report spammers than locking a thread.
This is possibly what makes others also leave their threads open after they have the answer they are looking for because they are expecting certain users to comment on their threads. You expect those you call experienced to post in it when you know you should be locking it and that leaves room for spam posting whether you admit or not. Sometimes the experienced users you want may not even be knowledgeable in the field that question is on.