I have a theory that the advent of the mobile phone has contributed a lot to feet on seats. People want to use them supported at a comfortable viewing angle, rather than hold them aloft with their feet on the floor and thighs horizontal. Elevating legs with the knees bent provides the perfect resting position for their phone.
It's no excuse but it makes some sort of sense if you've ever tried to pay attention to a phone while in a non-table seat. There really is no explanation for people at table seats using the facing seats as a footrest though. Another reason I always choose an airline seat, but even then you get the youngsters leaning against the window with their feet all over the aisle seat
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