If third-party app stores are allowed, companies like Epic or Facebook might just make their own app stores (or "launchers"), like they can on Windows. Governments could disallow the App Store and instead use a country-specific app store with apps that compromise privacy and include other user-hostile behavior. While it's arguably the government's responsibility to legislate and enforce privacy regulations, Apple seems to be much better at this than I would trust a government to be. Having the single App Store on iOS devices is easier, more intuitive, less susceptible to malware (and it's easier to remove malware that gets through), less susceptible to extreme government intervention, and better at protecting user privacy.

By using an Apple device, you trust Apple. They design the hardware (including the processor and Secure Enclave), write the software, and run iCloud. Trusting other companies to build their own app stores in a privacy-preserving and user-friendly manner—especially when their business model involves selling user data—seems decidedly user-hostile.