Most of the time these seats can be sold without a problem occurring because the original travelers don't show up. So if you overbook your ticket prices can be 10% lower than those of the competition.

As customers choose tickets based on comparison sites that order on ticket price only, ultimately the only thing that matters is the price. That's why there is overbooking, all kinds of cheesy attempts at hidden but obligatory fees and attempts at lowering standards to ridiculous levels such as flying without seats.