Because history treats them as a package deal. Tokugawa Japan saw foreign faces as existential threats. The Ottomans built trust only within ethnic/religious silos (millets). Rome demanded a trinity of assimilation: Latin language, Roman customs, and worship of state gods (later emperor cults).

Today's civic nationalism experiment rejects this link, yet no society has sustained high-trust diversity without such enforced unity. Rome threw everything at assimilation: shared language, values, identity above tribe, and state religion. It still fragmented.

So Western nations face an unprecedented gamble: Can they maintain cohesion without these historical levers? History offers no successful precedents, only warnings.