The system is built around expected no-shows
Airlines know that a certain share of passengers on many routes will not board because of missed connections, changes, cancellations, or voluntary no-shows.
Overbooking is an attempt to match sold inventory to expected actual usage.
Why it exists
From the airline perspective, an empty seat on departure is usually lost revenue. So the system tries to reduce that waste.
Where the tension comes from
The model works until more passengers show up than expected.
At that point, the airline has to reassign the problem through volunteers, rebooking, or denied boarding processes. That is why overbooking feels random to travelers even though it comes from a probabilistic operating model rather than a one-off mistake.