Refunds and no-shows
True, if passenger tickets include the flexibility of not turning up without notice to airline and afterwards rebooking or refunding the ticket, then it is rather difficult for the airline to cater for people wanting to fly when they deem fit.
Therefore, a part of no-shows are attributable to passengers who are irresponsible/make full use of the flexibility provided with the tickets. Maybe a large part of no-shows.
But even if missing a plane means forfeiting the ticket clean away, there will be some passengers who cannot show up or inform the airline because of unavoidable ill fortune. Therefore, the question is, if a flight is sold out and there are people needing to fly left over, what to do in the likely chance that some passengers miss the plane? Every missing passenger means another passenger who does not get the opportunity to fly.