It would cost too much. Commercial airplanes are designed to keep as little weight on board as possible, because this saves fuel, increases speed and profit margin. You may have noticed if you've flown that every single part of an airplane has been ergonomically designed to use the least space and is composed of the lightest-weight materials possible. Specs of the small Boeing 737 typical operating empty = 42,895kg The three-point landing gear on this unit is probably over 3500kg making it a major weight component. The landing gear is composed of high quality alloy and carbon fiber components and still takes up a significant portion of overall weight. Landing gear on commercial jets is very simple, operating on electric motors driving gears which raise or lower the three sets of wheels. If there is no power or a mechanical failure, they can easily be operated by hand by the crew from inside. It is one of the least likely systems to fail on a commercial airplane. In addition the fuselage would have to be enlarged and the cargo and passenger/cockpit areas shrunk to fit an extra set of any kind of gear, and there would be an additional cost in the electrical and mechanical control mechanisms for it. Landing gear failure is a very rare problem that is very well publicized when it happens.

Mainly because the failure rate is so rare that they don't need them. Landing gear is very reliable, and very strong. The struts and oleos (shock absorbers) are usually cast from solid aluminium alloy, and are driven up and down by either hydraulics, air or electric motors. Landing gear is designed to free-fall into its down position in the event of a hydraulic failure. They are designed with over-centre locks and other various down-position locking devices. The previous answer is quite correct in respect to the bulk and weight of adding extra landing gear. Landing gear is attached to extremely large structural beams in the body of the aircraft (in fact, apart from where the wings join on, these are probably the single strongest structural component of the plane). They are designed to withstand landings far in excess of the norm. Obviously there will be the occasional failure, but very rarely is it ever a structural failure. Usually it will be a failure of an associated system - such as hydraulics - which prevent the landing gear from locking down properly. Multiple-wheeled units are designed to be stable even if several of the tyres blow out, and the brakes are designed to be able to operate at high capacity even when red hot, without catching fire. So apart from the obvious issues of space and weight when it comes to adding extra landing gear, the main reason they don't add extra landing gear is simply because they don't need it.

Only one airplane I've ever worked on had an auxialiary landing gear. The BAe HS125-800 Hawker. Along the centerline of the fuselage there is a built in skid designated in the maintenance manual as the aux gear. Speaking of 737's, don't put the chocks in the wheel wells. The gear will go up but they won't come down.

1. Landing gear has a very good safety record, and a backup really isn't necessary. 2. The added weight and fuel economy hit would be high. 3. The engineering difficulty would be great.

The landing gear on heavy aircraft take tremendous loads but do so reliably. What makes you think they would need it?