Because flights are overbooked. The Airlines know unfortunately that some of their customers won't or can't show up. Those seats are assigned to standbys which are people booked without seats.

In 2026, airlines assign seats at the gate primarily to manage Basic Economy passengers, standby travelers, and weight-and-balance logistics. When you book a "Basic Economy" ticket, you are not guaranteed a seat at booking; the airline holds a block of seats to ensure they can accommodate families or elite members who might book last-minute. By waiting until the gate, agents can "puzzle-piece" the remaining passengers into whatever middle or rear seats are left. Additionally, for smaller regional jets, the gate agent may need to move passengers to the front or back of the cabin to ensure the plane's center of gravity is within safe limits for takeoff. Gate assignments also allow airlines to process "standby" passengers—such as employees or travelers from missed connections—only once they are certain that "confirmed" passengers have not shown up. This "just-in-time" seating strategy maximizes the airline's "load factor" while giving them the flexibility to handle the unpredictable nature of modern air travel operations.