Why don't incandescent light bulbs last forever?
Don't hang on, Tom—nothing lasts forever but the Earth and sky. Still, I suspect you want me to put down the bong and answer your question.
First, we need to understand how light bulbs work. You probably have a hazy memory about Thomas Edison rolling a cotton thread in soot and using that thread to fly a kite in a thunderstorm, thereby inventing the steam engine. (Hey, I said it was hazy.)
The story's emphasis on Edison's long search for the perfect filament makes it seem like there's something magical about carbonized cotton (or the more recent tungsten) that causes it to glow when a current is passed through it.
There isn't. The current is just a way to get the filament hot—and any material, from iron in a forge to, oh, I don't know, a lump of burning marijuana—will glow when it gets hot enough.
The trouble is, most materials don't last long at white-hot temperatures. This is great when you're seeking the chemical inspiration for "Dust in the Wind," but not so handy when you're trying to find your keys in a darkened room.
Edison slowed the burning process to a crawl by putting the filament in a vacuum: no oxygen, no oxidation, aka burning.
Nowadays, we fill our bulbs with an inert gas, like argon, that won't react with the filament material, so the filament will last a pretty long time. Even so, in all the white-hot molecular excitement, some tungsten atoms do fly off—you can see them as a darker deposit on the white inside surface of an old light bulb.
Eventually, enough material is dislodged from the filament to make a spot too thin to accommodate the current, and—to use the scientific term—poof.