More often, the profession of editing chooses them. In one sense, everyone edits. Writers who revise even one word in a manuscript have started editing their work. A speaker who pauses in mid-sentence to find a better word or phrase, a manager who revises an instruction or restates a policy, a secretary who clarifies a sentence in someone else's dictation is, for the moment, an editor. In another sense, editing is a highly specialized profession.
Most people become professional editors because they are intrigued with language.
They can't ignore mistakes in newspapers. They wince at broadcasters' blunders and cringe when illustrations don't match the accompanying text. A poorly constructed or illogical argument, or an inaccurate use of statistics, offends them. They have a compulsion to impose order on chaos, to create coherence and consistency. From every sow's ear, editors try to create a silk purse.