Avoidance learning offers one good explanation for why phobias are not ordinarily extinguished without treatment.

Avoidance Learning

a type of learning where a subject learns to avoid a negative situation or stimulus by performing specific behaviors.

Phobias

Intense, irrational fears of specific objects, activities, or situations that lead to avoidance behavior.

Extinguished

In psychology, extinguished refers to the process by which a previously learned behavior diminishes or disappears as a result of non-reinforcement.