Why can't other companies emulate iOS?

They can: https://emulation.gametechwiki.com/index.php/IOS_emulators

The issue is trying to make a business out of it.

The issue is violating Apple trademarks. Apple vs Corellium ended with Corellium successfully arguing Fair Use on a novel iOS emulator that they sold to researchers. Apple's only protected trademarks were for the wallpaper and icon art.

Conditionally, you are allowed to make a business out of selling iOS emulators.

> As to count one, we agree that Corellium is shielded by the fair use doctrine. First, Corellium’s virtualization software is trans-formative—it furthers scientific progress by allowing security re-search into important operating systems. Second, iOS is functional operating software that falls outside copyright’s core. Third, Co-rellium didn’t overhelp itself to Apple’s software. And fourth, Co-rellium’s product does not substantially harm the market for iOS or iOS derivatives—so Apple’s own incentive to innovate remains strong.

https://law.justia.com/cases/federal/appellate-courts/ca11/2...

Why can't they build it?