I'm not a teacher, but I would suggest that it's because airline seat are thought of as containers. We also tend to say "in" of airline seats, rather than "on". So while we "get off" a seat at home, we get "out of" seats in planes, buses, trains, etc. All are narrow, confined and resemble containers for human cargo more than actual seats. That's my take on this one, anyway.
Student or Learner