Remember that there are millions of people in Southern California - and most of them do NOT have Annual Passes to
Disneyland, but instead make a day trip to Disneyland every year or two. Those are the people that are there on weekends buying single day tickets.
I think that AP holders usually buy a blockout day ticket only if there's some special event going on that they want to attend, or if they want to join non-AP friends/family who are going on a blockout day.