You can't jump out of a crashing commercial jet. The decline can be very steep, for one thing, and unpredictable for another. At any time the plane could bank sharply, flip, or change trajectory (there is an aviation term for this, but I can't think of what it is right now). You run the risk of being sucked out of the plane, getting hit in the head or body (HARD), or being sucked into the engines. And that's just a few of the dangers on the outside of the plane!

On the inside, fighting panicking people or waiting in line for your turn to jump would render the parachute ineffective. That being said, flotation devices are pretty pointless as well. But they're better than nothing. Interesting question!

The above reasons and because Skydiving is more than just yelling Geronimo.

If the pilot can maintain control long enough, at a low enough airspeed for people to stampede each other queued for the doors, well then he or she can maintain slow flight long enough to make a safe landing.

It's also unfortunate when someone accidently snags a D-ring and the victim isn't at the door yet. The chute will find it's way out of the door while the cords can slice the aluminum fuselage open like a hot knife through butter while the chute pulls the cadaver through the empenage.

As Anonumous says http://www.answerbag.com/a_view/6520221

Also it is quite rare for planes to actually plummet to earth entirely out of control. You plan for the likelier event first.

It's safer to land on water.
Planes are designed to do that as New Yorkers saw recently.