Commercial airplanes fly high and fast.
As mentioned above, skydives are meticulously planned, and the height of the jump is one of the most crucial details. The highest point a skydiving plane would go is 15,000 to 16,000 feet. Commercial planes, on the other hand, fly on an average height of 35,000 feet above the ground. At this altitude, the temperature drops down significantly and breathable air becomes scarce. Even if passengers manage to jump out of the plane safely, they would have to be wearing HALO, or high-altitude equipment, comprising of an oxygen cylinder, regulator, mask, altimeter, ballistic helmet, and flight suit. Without these, they might pass out due to a lack of oxygen.
The speed of commercial airplanes is another aspect that makes skydiving from them almost impossible. When cruising at a high altitude, the Boeing 737 can reach a speed of 600 miles per hour. Air traffic controllers typically assign the speed and altitude, and longer flights tend to fly higher than the average altitude of 35,000 feet to 39,000 feet. Jumping from this height, when the plane is flying that fast, can even itself be fatal!
Parachuting gear is expensive and bulky.
For starters, even a single parachute would be too bulky to fit under the typical seat in the economy class. It is also quite heavy. So, airlines would have to come up with the necessary space to accommodate all the parachutes. If you have ever been on a plane, you already know how strict they are about the weight of luggage, especially carry-ons. Now, imagine adding one parachute per passenger. It would easily increase the overall weight by 6,000 to 8,000 pounds. Add the cost of that to the HALO gear we mentioned above.
The parachutes and the gears themselves are fairly expensive, and making space for them would require even more money. Of course, the airline would not pay for it out of their pocket. They would include the added cost to the price of the flight tickets which are already more expensive than other forms of public transport.
Finally, most accidents take place during takeoff and landing and not mid-air.
Statistically speaking, most airplane accidents take place during takeoff and landing. In fact, from 2003 to 2012, merely 9% of all fatal airplane accidents took place while the aircraft was cruising at high altitudes. Moreover, one of those accidents took place because of a thunderstorm. Parachutes are useless during takeoff and landing, but it can be even more dangerous to parachute out of a plane during a thunderstorm. Even trained parachutists would not attempt it.
All in all, having parachutes onboard commercial flights might sound like a good idea at first. However, when you examine the logistics, it does not seem like a very practical or even a viable proposition.