Reformers are just like entrepreneurs: They too are fighting against reality, trying to carve out space for their vision of how things could be better. Ms. Khan’s journey to confronting Amazon in federal court is in some ways an even less likely tale than Mr. Bezos’s. And so, like Mr. Bezos in the early years of Amazon, she has become a figure of fascination.

The daughter of Pakistani immigrants by way of London, Ms. Khan had the natural instincts of a good journalist. At Williams College, where she worked on the school paper, a friend described her as especially interested in understanding power, particularly the way it conceals itself to seize more power. She was in her late 20s when she wrote her paper on Amazon — about Mr. Bezos’s age when he quit his Wall Street job to drive with his wife at the time, MacKenzie Scott, west to Seattle and his destiny.

Antitrust law was the traditional tool used to rein in companies that became too powerful. Antitrust played a major role in the 1890s, marking the beginning of the Progressive Era, and again in the 1930s under the New Deal. But by the early 1980s, antitrust was at a low ebb. The so-called consumer welfare standard reduced antitrust down to one issue: the price customers paid. If prices were low, there was no problem.

The Microsoft case was important and influential, but it was very much an aberration. In the early years of this century, the prevailing laissez-faire philosophy allowed not just Amazon but other start-ups to rise much quicker than they might otherwise have. Facebook and Google charged users nothing, and were allowed to acquire their way to dominance. Six of the eight most valuable U.S. companies are tech companies — seven if you consider Tesla a tech firm.

Government was slow; Silicon Valley was fast. The marketplace would decide the fate of corporate empires. By 2015, when Ms. Khan was entering law school, hardly anyone was interested in promoting competition through government intervention. Criminal justice reform, environmental law, immigration — those were the topics that appealed to students. She chose antitrust, practically alone.