Carpe Diem: it's OK to resign! [,,,] why drag things out?

It might surprise you but for some people, it's not. They'd rather play the loss than resign it. It may or may not be related to suicide versus natural death. ;o)

Another possibility is that the person thinks that they're doing you a favour by giving you the satisfaction of the final, winning, move. I do know that I sometimes feel robbed of that minor pleasure when someone resigns close to the end.

A third possibility is that "mathematically eliminated" does not always occur to the non-mathematically minded.

A fourth ... an opponent might be resentful of the loss and dragging it out to punish you.

The most likely, I would guess, is thoughtlessness or mindlessness; the force of habit.... The game appears, the player moves, on to the next....