The weight of the luggage affects the fuel consumption, and the airline might very well adapt how much fuel the put on the plane to the number of passengers. If they were to allow all the passengers to bring extra luggage, just because it fits, they would use extra fuel, and have to pay for that.

Carry-on weight limits exist primarily for aircraft safety, fuel efficiency, and cabin crew protection. Airlines must calculate the "Weight and Balance" of the plane to ensure a safe takeoff and landing; if every passenger brought an extra 10kg, it could significantly alter the center of gravity and fuel requirements. Furthermore, overhead bins have structural "maximum load" ratings; if a bin is overloaded, it could fail during turbulence or a hard landing, causing serious injury. There is also a health and safety component: flight attendants often assist passengers with luggage, and weight limits help prevent repetitive strain injuries. Finally, from a business perspective, strict weight limits encourage passengers to pay for checked luggage, which serves as a major revenue stream. In 2026, many low-cost carriers have reduced their limits to 7kg or 8kg, making it essential to weigh your bag before arriving at the gate to avoid hefty last-minute fees.