Commercial airliners typically do not have external fuel tanks for several reasons:

1. Structural considerations: Commercial airliners are designed with careful consideration of weight distribution, aerodynamics, and structural integrity. Adding external fuel tanks would significantly alter the aircraft’s weight and balance, potentially compromising its performance and safety. The aircraft’s structure would need to be reinforced to accommodate the additional weight and stress, which could lead to significant design changes and certification requirements.
2. Safety concerns: External fuel tanks introduce additional risks in terms of fuel leakage, damage during flight, or separation from the aircraft. These risks could pose safety hazards to both the aircraft and people on the ground. Commercial airliners are designed to maintain a high level of safety, and adding external fuel tanks could increase the complexity of fuel management and introduce additional failure modes.
3. Fuel efficiency: Aircraft manufacturers strive to maximize fuel efficiency, as fuel is a significant operational cost for airlines. Designing aircraft with internal fuel tanks and optimizing fuel systems helps to reduce drag, improve aerodynamic performance, and enhance fuel efficiency. External fuel tanks could increase drag and fuel consumption, resulting in decreased efficiency and increased operating costs.
4. Operational considerations: Commercial airliners are primarily designed for passenger comfort, efficiency, and operational requirements. The need for external fuel tanks is typically not warranted for the types of flights and distances commercial aircraft typically operate. Internal fuel tanks provide sufficient fuel capacity to meet the range and operational needs of commercial flights without the need for external tanks.

While the concept of external fuel tanks dropping off during landing may seem intriguing, it is not practical or necessary for commercial airliners. The design and operation of commercial aircraft prioritize safety, efficiency, and reliability, and extensive testing and certification processes ensure that these standards are met.