Overbooking vs. waitlists

Why do airlines overbook?

The best reason is that since usually some passengers do not show up, overbooking enables the airline to let passengers fly who otherwise would have been turned down as the plane was sold out already.

But when the airline then overbooks, and more passengers show up than expevcted, what then about the original passengers? The sense of the notion of overbooking is that all booked passengers are equal. Thus, overbooking puts the existing passengers deliberately at a jeopardy.

Would it not be more logical, once a flight is sold out, to respect the existing passengers and to put the others who wish to fly on waitlist of some sort, so that in case of a full aircraft, all the original confirmed passengers are assured of places?