Many of the aircraft that call for putting the door on the seat have inflatable slides on the wings (due to wing height or other reasons). I certainly wouldn't want to be the one who tossed a door out the window when told not to, damaging the slide, landing on the ground in a place that presents a hazard (remember it could be smoky and hard to see), or smashing something on the wing that becomes sharp and injures another passenger. Or the one who "places it on the floor" behind them blocking dozens of others from escaping. An innocent mistake under pressure is one thing; deliberately ignoring instructions and hindering others because you just decided you knew better is another. Remember there are dozens or more people behind you trying to escape through the same exit. On the armrests of the seat as shown is likely the best "somewhere out of the way" for that particular plane.

The aircraft was presumably certificated and evacuation tested following the safety card -- it seems prudent if seated in an exit row to study it, note the steps and things that differ by aircraft like where to place the door, and visualize yourself doing it that way so that in the unlikely event of an emergency you can do what's called for.