Because flights are expensive enough that for most ordinary people missing one would set them back years or decades financially?
Ok, now take that figure and deduct tax, housing, food, utilities and so on - how much do you think is disposable/saveable? And then take the typical cost of a last-minute replacement flight and compare those two numbers.
it would be incredibly inconvenient, and maybe missing other parts of a full vacation would set them back, but thats not the only reason people buy flights