If all pax who booked turned up for every flight of every day of every carrier - then there would be no overbooking! It is other people to blame, not the carriers as such.
The carrier is entitled to make the maximum amount of revenue they can. They balance the denied boarding against the empty seats and plan accordingly. Upsetting people with denied boarding is not in a carrier biz plan, so they will always seek to minimise.
It used to be that one 'confirmed' the flight a day or two before departure, particularly for long haul. However, carriers found that in recent 10/20 years, this made little difference to the no-shows. So they saved themselves the cost of having to reconfirm. Turning up early for check-in is the most certain way of getting on the flight!
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"I tell you, we are here on Earth to fart around, and don't let anybody tell you any different." Kurt Vonnegut, Jr.