Because we're stupid and live in a world totally controlled by greed.
Because they're the cheapest, most plentiful, most convenient and most reliable sources of power available. The United States has millions of tons of coal available that are being underused because of pollution and inaccessibility concerns.
The only other "renewable" sources of energy (for now, anyway) are wind and solar power, and they're both expensive, unreliable and not nearly plentiful enough to sustain the amount of power required.
France offers an excellent example for the world, producing roughly 80% of its power through nuclear fission, but the improbable (although remotely possible) chance of rupture/leak of the containment building dissuades many other countries from taking advantage of this source of power.
Because big business wants to suppress the truth about hemp fuel being far superior to oil.
Because, for now at least, they are the only energy source that can give an acceptable return on the input. Go invent cold fusion, I'll switch to it faster than government can put a tax on it
Because they are cheap and easy.
They are realitively cheap, abundant, low tech, and our society is set up to run on them.
because the big oil companies say so