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Why did Democritus believe that there must be infinitely many worlds?

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Bronze bust of Epicurus with a full beard and draped clothing, displayed on a round inscribed base at the National Archaeological Museum of Naples. — Atomism Epicurus, bronze bust from a Greek original, c. 280–270 bce; in the Museo Archeologico Nazionale, Naples.

Democritus believed that there must be infinitely many worlds because he posited that both atoms and the void (empty space) are infinite in number and extent.1

In his mechanical system, motion is eternal and occurs by necessity rather than by design. Because atoms are infinite and have always been in motion, they inevitably collide and unite through a whirling movement to form larger bodies and worlds. Consequently, Democritus concluded that there must always have been an infinite number of worlds, all consisting of similar atoms in various stages of growth and decay.1