In the early 1980's, a group of Santa Cruz physics grad students built a set of computers into their cowboy boots. These timed the spinning of roulette wheels and applied Newtonian physics.

Thomas Bass wrote this up in the 1985 book, The Eudaemonic Pie, and caused the Nevada Gaming Commission to ban the use of these devices.

See http://en.wikipedia.org/wiki/The_Eudaemonic_Pie

When a satellite fails, often it cannot be de-orbited. Several failure modes will cause this - the most common is the malfunction of the controller, communications unit, or onboard power system. When any of these fail, there's no way to command the retro-rocket to fire.

Then, too, you need the satellite to be pointed in the correct direction (meaning that its stationkeeping rockets are working), and for it to have enough hydrazine (or whatever) to be deorbited. Near the end of a spacecraft's life, consumables are limited.

And, of course, it takes a lot of energy to de-orbit many satellites. A geostationary comsat needs one heck of a kick motor to get it down. Usually they are not brought down to burnup in the atmosphere. Instead, they are moved a few dozen (hundred?) kilometers inwards from their geostationary slot. This puts 'em well away from the main circle of geostationary satellites.

It's like consumer goods ... manufacturers work to make them last long enough to complete their mission; few think about how to get rid of 'em once their purpose has expired.