The Southworst effect definitely has something to do with it, but I also think people are really just dumb. Most infrequent fliers genuinely believe the seats are all the same, and even though it is self-evident the exit row is better there are other seats that at a glance seem the same such as premium econ areas which are not clearly deliniated. I had a confrontation with someone recently over this. I was in row 12 on AA, which was the last of the MCE rows. The guy was supposed to be in row 13 but "mistakenly" sat in 12. When I confronted him about it and pointed to the number he said "Well there is hardly a difference, is there?". I had to explain the MCE vs Regular Econ thing....although, I kind of suspect he already knew.

The other factor is flat out self interest and entitlement. They simply like your seat better. I have had incidents with exit rows where the second I point out they are in my seat, the get up without a word and move to their own. No argument..but no apologies either. Simply chancing it I guess. Others simply didn't feel that they should pay for seat selection and simply took the seat they want as a stick-it-to-the-man form of protest. And still others are simply just willfull and will put their intrests before yours and fight for it.

Nobody, and I mean nobody, poaches my seat and gets away with it. Heck, I say no to swap requests too. I have split up couples and parents from teens as a result of poaching. Want to sit together? Pay for it. I did.