I assume you mean larger commercial aircraft. Most commercial planes have enough seating for at least 100 passengers and the number of emergency exits varies on these planes. In the event of a sudden and catastrophic emergency there would likely not be enough time for all the passengers to exit the plane via parachutes through the emergency exits. So, parachutes would not do much good. In most cases, it would be useless as there is simply not enough time to react to such an emergency. Economy. Parachutes require regular checks and maintenance/replacement even if they are just stowed away. Passengers would have to get an even lengthier safety briefing pre-flight and special training of air crews,to instruct passengers, would also be necessary. All of this is costly. Some smaller aircraft do equip them. As a matter of fact some even have parachutes for the airplane itself (this is relatively new and extremely rare). Flying magazine did an article on this last year. Keep in mind when I say small aircraft I mean 2-6 passengers/crew. This is the best answer I can come up with. I'll leave the more complex answers to those who have a better understanding of what conditions (altitude, etc) need to be in place for a successful jump from a commercial plane. In the meantime to shed some additional light on this very complex question: From http://www.super70s.com/Super70s/News/1971/November/24-DB_Cooper.asp DB Cooper: "The original jumping skyjacker, Cooper boarded a Northwest Orient flight in Portland, Oregon on 24 November 1971 -- the eve of Thanksgiving. Once aloft he threatened to blow up the plane and demanded $200,000 and four parachutes. After the plane landed at Seattle-Tacoma Airport and his demands were met, Cooper ordered the 727 to take off and head for Mexico. He jumped from the rear of the plane somewhere over Washington state, taking the cash with him. Despite exhaustive searches, Cooper's body was never found and his whereabouts are unknown. The money and parachutes were delivered to the plane and Cooper then demanded to be flown to Mexico. Knowing that he would be jumping out of the back of the plane, he also demanded that the pilot, which was actually co-pilot William Rataczak, fly with landing gear down, flaps set to 15-degrees, to not exceed 170 miles per hour and not to fly above 10,000 feet." (There had to be a good reason for such a specific demand about the speed and altitude and as you can see the plane was a Boeing 727)

The cost factor to the airlines aside, lets say their management decided to give it a try. Even if they equipped their airliners with them, the idea wouldn't work. The average Joe Passenger could kill or seriously injure themselves trying to use a parachute without any experience with one. Having been a skydive pilot for several years, even those who are PLANNING to make a jump have enough issues! There are many reason why this idea won't fly including: The aircraft must be going slow enough to allow the doors to be opened and a jump to be executed. The way airliner doors are designed, it's prohibitively difficult to impossible to open them inflight, even if the cabin pressurization were dumped. Using a "rig" (parachute) is not as simple a strapping it on a jumping! One must be trained on how to properly wear and use the equipment. First time skydivers who do not do a tandem jump (strapped to an instructor) but jump solo must take Accelerated Free Fall classes of about 8 hours worth of instruction on parachute operation before they make the jump so they are versed in how to use the equipment. Even then, jumpmasters exit the aircraft with the AFF student and are attending them during freefall until they deploy their chute. AFF student also wear radios so they they can be coached through their canopy maneuvering and into landing as necessary. There are any number of 'malfunctions' that can occur during a jump. The jumper must be aware of and know how to correct them when they happen. The chute may not inflate properly, the lines could become tangled. If it's a bad enough malfunction, cutting away the main chuteand deployment of the reserve chute may be necessary.Think they could figure out how to do that? Landing is another problem. Unless the jumper is experienced enough to know how to control the canopy and steer their way to a proper landing site, they could impact any number of structures. Even if they were lucky enough to be over an open field, they need to know how to 'flare' for landing or they risk serious injury. Also, can you imagine 100-300 people rushing the door just trying to get out?! These are just the main points why parachutes for airline pax are an unrealistic but I think you get the idea. For more information about skydiving, visit the United States Parachuting Association at www.USPA.org

The really really short answer is: it's pointless. 90% of commercial aircraft operate in settings (altitude,speed,temp) that would kill you pretty efficiently.

Just as bjminla has stated, The altitude at which commercial Planes fly means the oxygen is very thin thus the capability of breathing is almost impossible, and death would be almost certain. But If the choice was to stay there and die or try your luck with a parachute, I know which Id choose.