Why do airlines regularly overbook flights?

What do you mean "happens so often"--how often have *you* been refused passage? This is one of those "happened to a friend of a friend" scenarios.

If they didn't overbook, they'd have to raise ticket prices to cover the cost of the empty seats they'd have to leave open. Would you rather pay more for your ticket all the time, or have the infinitesimally small chance that you might have to take a later flight (and get compensated for doing so)?
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