Believe it or not, I know people who prefer middle seats.

More realistically, if someone has an onward connection (and it need not be another aircraft service) they may want to do everything they can to get off quickly.

I also refused to move from a middle seat once, when it became clear that it was a married couple try to block off row 1 for themselves. Long story, unrelated to this situation, I'll leave it for another day!