Well as someone who has been a single flyer, they may have had their reasons for not moving. I always pick or pay for an aisle seat, as I get clausterphobic if I have to sit by the window, or between 2 strangers. Yet for some reason when I have been asked to switch, they always want me to trade my aisle seat for a middle seat near the back. No one has ever offered to give me the seat closer to the front, or an aisle seat.
This will always be the case. Everyone seems to have the attitude of "they are by themselves why don't they just move" However if your to the point where you can't find two seats together that means there are only middle seats left, some people don't want to sit in the middle seat. I have only had two do this one time when at least one of the people next to me was not related to me and its not something I want to do again (and I still hate it if both aren't people I know). Others just want to get on with the flight, and its a huge pain to get up from where your sitting and move your overhead luggage and whats under the seat to the new spot. Plus the person who is willing to move has to either be next to someone else also willing to move or next to one of the few still open seats. If not they really didn't help you any.
Now for a small child that go there late due to a connection issue I may move, however there is no way I would move for two adults even if they were on their honeymoon.